Sets.
However, I did order a very large amount of ball joints and sockets for mecha recently from BrickLink.
Bricks and minifigures because they have a large bin full of bionicle parts and tons of random pieces from incomplete sets
A bricks and minifigs just opened up near me and now I can’t find any lego deals on craigslist or goodwill. Ten years ago I used to find collections pretty easily but then the resellers moved in and bought up supply. Now if I need parts I just buy gobricks from china or use lego’s online pick a brick service. Out of curiosity I’ve asked random moc people at b&m if they’ve ever heard of gobricks. Usually nobody knows about them or they tell me they only buy first party lego because they’re better quality. Well I can tell you, when I was a kid twenty five years ago there were no good alternatives to lego but times have changed.