Depends on where I am at and what I need.
Early on I try to get a few essentials. I need a supply cloths bolts woven from plant fiber, wool, and silk, just in case a dwarf in a mood gets particular about something. I have discovered that whatever you are not producing at a given time is likely to be what a dwarf in a mood demands ("Urist's Law of Moods" or something like that) so I try to make sure I have the bases covered.
I will also trade what limited things I can produce early for what other things I tend to consume more of than I produce in the early game. For example, I often ask for raw ores I need, flux stones, and coal and lignite to make fuel. If they have items made of metals I need but I do not particularly care for the item (and it is unadorned and cheap) then I might buy it off them so I can melt it and recycle it's material. If I lack fire clay then I import that so I can produce pottery that does not require glazing. Sometimes I just take clay off the merchant's hands anyway because it is cheap and heavy, and frees their wagons and draft animals up to haul off more of my stuff. Same thing goes for cages. Wood is often useful to save me the effort of chopping down trees and hauling all the logs from wherever the tree was. Leather is a big one, since it is such a versatile resource but my own hunters and ranchers tend to produce less than I would like without cluttering up the fortress with bones and skulls.
Beyond that, I tend to buy a lot of food and drink, the more exotic the better. I will often sell high quality prepared meals in exchange for simple staples I can turn into yet more prepared meals. Dwarfs get happy thoughts from having a varied diet, and dwarfs with particular preferences are more likely to have it met if a wider selection is available. Also any above-ground vegetables I lack in my native biomes I can turn into gardens I could not have had before.