I really hate (items), i.e. stuff that's brought in from outside, so generally don't buy much, and even then pretty much only raw materials that I don't have locally available. I aim to be very self-sufficient, and quite realistically for a pseudo-medieveal setting, only trade for stuff I can't produce myself. Leather isn't listed because I've done the "skin as glob" raw edit which gives you multiple pieces of leather from larger animals (and balanced it out with tanning needing 3 bits of skin for 1 "leather", so eg. water buffaloes give 4 leather for me, pigs 2, eg. cats or poultry should still be only 1).
- Might buy food/booze, or at least brewable plants, from the first caravans, if I've been slow with farming. If I end up doing the edit to lengthen growing times, this will be important/necessary for longer. The "settling in" phase.
- Seeds if I don't have the plant yet, or have used up all seeds of a type by accident (at least for underground crops).
- Initially some sand/clay for moods if the map doesn't have them
- Wood if the biome really necessitates it for beds (although I should probably just make a habit of hitting caverns quickly in such cases), or I'm using a lot of charcoal (ditto regarding caverns for wood, or even better, dig deeper for magma)
- Important ores (copper, tin, iron) or flux if it's not available locally. Other ores like gold, platinum only if I want to have it available for moods or something.
- Possibly kobold bulbs and valley herbs for refining.
- Animals, both exotic and domestic, if it's something I want.
- gypsum and/or gypsum plaster powder, until I have enough of a stockpile.