What do you want to sell? Generally speaking, grocery stores in various cities can sell things, spice shops can only sell medicines and alchemy materials, weapons shops and blacksmiths can only sell weapons and equipment, and clothing stores in Dugu can sell clothes and jewelry, and there are booths in five major cities. Hotels can sell food. If it is stolen, in addition to the woman who gave you a thief armor with a hip flask, you should also do the task of unlocking the stolen NPC, including the Kartman caravan outside the city, Nilanya who set up a stall with a helmet, and so on.
There is also a classic black market of DLC dark magic, which can summon a magic dealer. You may meet vendors at random on the road, but you don't have much money. In some places, you can meet hunters and trade the meat and fur obtained by hunting.