trader mostly sells QoL stuff. you don't really need it, but it makes life easier:
flesh of ymir is an ingredient for crafting frostner(one of the best weapons against bonemass and yagluth) and iron sledge (if you're into 2h hammers this thing is awesome. treat it like an upgraded stagbreaker).
dwerg circlet gives you light around you. yes you can build a campfire or increase brightness in the settings instead, but this thing is just more convenient.
and the belt that gives you 150 more max weight. must have thing for mining ore. cuz this way you can bring more in one go)
thunderstone is an ingredient for oblitirator: a way to destroy all of your unneeded items you hoard in your chests (and maybe get some coal in a proces). especially useful for useless trophies. 1 coal per trophy.
fishing rod and fishing bait can allow you to fish. not sure it's that useful, but fish wraps are good (70hp 23stam for 2cooked fish and 4 barley)
and yule hat is for decorations only. i think.