In English, you don’t need to say “the open market at Tsukiji”, you can just say “Tsukiji open market”.
For the same reason, in Japanese, you don’t need to say 「築地の場外市場」, you can just say 「築地場外市場」.
This actually happens a lot, where they’ll just string a whole bunch of kanji together, and it can be hard to know where the word breaks are.
Just to add to that one, the の particle isn’t always about posession.
It is also used generally to modify nouns with descriptors and adjectives.
This is why 東京の大学 doesn’t mean “Tokyo’s university”, but rather “a university in Tokyo”, and doesn’t refer to a specific university. (I’m sure there are many universities that happen to be in Tokyo)