Can someone help me disambiguate 材木 (ざいもく - wood) and 木材 (もくざい - lumber, wood) and decide which is appropriate when?
Or if they’re basically the same thing, then… good 
This is one of those questions where Google Image Search can help a lot - just stick each word in and see what you get. 材木 gives me lots of images of raw logs, while 木材 is primarily wooden boards.
Wikipedia is also useful here - the page for 木材 comments that 材木（ざいもく）も同義だが、これは建材や道具類の材料などに限定する場合もある。
木材 is the broader word. It’s kind of a rectangle and square situation.
パルプ, as in what you’d make paper from, is not made from 材木. It’s made from 木材.