I have a Notion database of all the books I own and whether I’ve read them or not. When I want to buy a book I open the sheet and sort for ones not read. It’s a three digit number.
Sometimes looking at this helps and sometimes… I really want a new book 
I can’t hold myself back from buying too many books in English - I stood no chance with Japanese
Everything I’ve bought on sale has been things on my wishlist that I wanted and would have been happy to pay full price for if I had waited until I had time for it.
So sadly that wouldn’t help me lol
(For free books though I’ve definitely gotten stuff I would not ever get otherwise, but it’s free so who cares)
Definitely you should thank them.
I bought two books that are normally about 1050 for only 665 each, got coinback, used the coinback to get a third book in the series for free (and got coinback on that too since you don’t have to spend cash to get coinback, and this was one without a minimum amount required)
So 3100 yen worth of books for about 1330 yen while having a spare 332 yen for a future purchase.
And it just so happened on my wishlist there was a 299 yen book that is normally 1650 yen. Gotta love the wishlist.
(Yes I have a problem, but actually no I don’t lol)
i buy way too much shit but it’s all comfortably within budget so it’s not a huge issue. i’m sort of more a bargain hunter in any event but trying to scale back on the buying cheap stuff you’ll never read. i mostly don’t impulse buy below 100y these days unless i already know i definitely want to read it
i used to have this mindset, but after growing used to financial security, it turns out gifting things to people or having some flowers around once in a while is nice. (Though frugally speaking, growing the flowers yourself is both cheaper and a nice hobby)
I leaned fully into the 積読 lifestyle for several years, and I probably actively contribute to exacerbating it in other people more than most anyone through taking over the bookwalker freebies thread (which I have very mixed feelings about).
I don’t know if that means I’m a good or bad person to listen to, but it does mean I have rambling thoughts about the topic!
My main line of thinking around it is that - when buying stuff for yourself in general, there’s going to be purchases that turn out to be a dud you never get anything out of ever, and there’s going to be purchases that through serendipity turn out to be a treasured experience you wouldn’t ever have gotten otherwise. For better or for worse, most of the time it’s basically impossible to tell which is going to be which before you buy something.
Which is just to say I guess that… you end up having to take a lot of the duds with the treasures if you want to keep rolling the dice. But I think it’s worthwhile recognizing that it’s not inherently wrong to be tempted by the chance you might really enjoy something in the future. If you have the money I think to some extent it’s a good thing to think of buying yourself a book you might like as worthwhile without begrudging if you ever actually read it, the same way it would be if you were buying it for a loved one. The chance you’ll enjoy it has worth, even if it’s not a sure thing.
That said, for Bookwalker particularly, there is absolutely no reason to feel FOMO.
As a general rule if something is on there, it’s widely accessible and will stay widely accessible for the foreseeable future except for things like limited time free-to-read listings or back issues of some magazines, which are all marked. Whatever you’re looking at, whenever you come back to it, it’ll (very very likely) still be there, or somewhere else like it.
And I would hazard that someone who only buys their next book when right when they’re about to read it (whether on sale or not) would quite likely still ultimately pay less than all but the most conservative of those in the library-building, sales-chasing mindset who pick what they want to read next out of the backlog of what they already own (especially at a language-learning pace of reading).
I think for me and my outlook both lines of thinking come down to… what I’m purchasing in the moment is really just what’s in that moment: the enjoyment of imagining a future where you’ll have the time to read that cool book, the feeling of getting a good deal, the element of “well I bought it so now I don’t have to think about whether I should buy it or not!”
Depending on the situation and the amounts involved, those feelings might genuinely be worth the money on their own, or they might be a sign I’m in an anxious mood and should take a walk or go to bed. But either way the book itself is arguably just a bonus, more grist for the backlog of stuff to pull from when there’s already more in there than I’ll ever have time for in a human lifespan.
So I suppose probably like with any kind of impulse control, if you want or need to cut back it’s about finding principles to focus on that you care about enough that you maintain discipline against that want for instant gratification. I’m often not too great at that… and I haven’t personally had much luck with specific rules since I know I can break them. But sometimes thinking about it this sort of way can help the right mindset click into place, I think. Like, “right now do I really want the feeling of having bought a book, or do I want the satisfaction of having stuck to my principles and saved up a bit more for XYZ.” Doesn’t mean that you don’t end up picking the former sometimes… but at least it’s a conscious choice, which is a step maybe at least.
Amazon’s point-back feature is probably not very useful when it comes to my spending habit.
“Oh, you are telling me that if I spend 50 bucks more I get to take 3% of that back as points? Sign me up!”
…
Oh god, there’s some point-back deal going on the entire site.
I have at least 2000 books. I stopped counting. I don’t have enough bookcases. There’s books on all kinds of topics. I’m so interested in everything and have done so many interesting things, I have the books to prove my interests. I am truly sorry for whoever has to deal with them all when I die.
Rules… to limit oneself from buying too many books?
Wait, is that a thing?!
Rules were made…
To be broken.
Thankfully for me I only follow like 3 manga titles in Japanese actively (1 weekly, 1 monthly, and 1 tankobon-only that comes up like twice a year) so I don’t really buy outside of that, sale or not sale. I top up maybe around 2k-3k yen every 3 months for manga ebooks. It can last longer if one of the titles have a hiatus or I scored a coupon.
Remember, sales and coupons are tactics for the company to make you spend more money. If you want to save money, sometimes the counterintuitive way is to not be too obsessed about getting the best deal. If sales or coupons makes you buy way more than you intended, or makes you spend impulsively, that can actually cost you more money in the long run.
What has actually helped me limit this was getting a Kindle Unlimited subscription. It’s about 6 euros a month, and I can ‘borrow’ up to 15 books at a time. As a consequence:
Ignore them, despite having accepted that money is lost. (Which is easier when there are digital.)
New books I want to read can’t be less important than old books I used to want to read. I don’t mind keeping 積読 alive. (Maybe I should mind a little more.)
Many manga are that I don’t want to buy first volume full price, but became that I waited for promotion even if I am willing to buy them full price. In short, marketing works, and not always as 積読.
Other than checkmark in Bookwalker, you can always mark “want to read” in Natively or in Bookmeter.
Half of all my books in Bookwalker are unread.
i also had a time when i took all the bookwalker sales i could get, but one thing i realised is, i love me a physical book. for manga bookwalker is okay (though if i really love the manga, i want to own it and i might end up paying twice), but for novels i get the physical book. if you don’t look at delivery cost and VAT you have to pay in your own country, some novels are dirt cheap in comparison to european book prices. for example コンビニ人間 is 630 Yen which ends up being 3,66 EUR 
Nearly all of my English language books are non-fiction. They’re always handy! Early this year I made a quilt from a book that I’ve had since 2007.
Japanese books aren’t too hard to resist now I’m living in Japan, because they’re accessible. I can always get one if I need it, and probably on sale too. The unread Japanese books I have are either weird ones that I was given- like my ones on the Sengoku Era- or they’re ones I picked up to push myself and haven’t been able to bite yet. My husband got me the full set of 魔女の宅急便 a few Christmases ago and I’ve barely made it past the first page.
In my country there is one bank which is the cheapest for buying stuff in foreign currencies and since there is no other foreign currency outside of Yen that I buy things in, I only use that account for Japanese stuff… so I basically transfer an amount each year that I’m willing to spend on Japanese and if it’s gone there’s no way for me to buy more. 
(I mean theoretically I could transfer more, but it hasn’t happened yet that I had to resort to that)
Okay, I’ve synced my wishlists as much as possible and like, the 8 manga currently on sale are less than the coins I currently have, and if I’d bought them yesterday while the coin back was still on I could’ve replaced some of those coins. They’re basically free books. Who doesn’t want free books?
(I did have to stop following the bookwalker freebies thread because weekly reminders of free stuff was also weekly reminders of sales and spending money and it just wasn’t good for my impulse control to be tempted into buying more every week…)
but I’m also very happy to have the Frieren set and The Green Yuri set (so far).