I buy pretty much anything nonperishable the caravan brings, with a few exceptions like ridiculously overpriced gem-encrusted ammo, clothing, etc.
I sell junk first, like catapult parts < exceptional, crossbows < superior, bloody, dirty, and deteriorating clothes, and anything worth under $100. Storage space costs labor, so the more you can concentrate and keep value the better.
Encrusting, sewing images, studding with precious metals, and other value-adds along those lines are a great way of saving space. A cloth image on a sock might not be as valuable as a second sock, but you don't have to find room for a second sock, either.
Prepared meals are a ridiculous bonanza, particularly when using ingredients with an extra stage of value-added preparation, like flour or quarry bush leaves. For the amount of labor required to get from rock nuts to $10,000 stacks of lavish leaf roasts you could probably make more mining and crafting gold, but you can't eat a golden trumpet, you're going to overproduce food anyway, and gold eventually runs out.
So I'll buy ~$20,000 worth of metal and gems, dump several tons of crap at $20-$100 apiece, then make up the difference with a couple of $8,000 quarry bush leaf roasts.
All the fine clothing and extravagant treasures my dwarves labor night and day to produce stay in the fort.