In general, when selling items, visit the traders in this order:
Therapist?buys: Barter items, Valuables, Meds, Keys
Skier buys: Attachments, Barrels, Clothing, Armor / Helmets, Bags
Mechanic buys: Guns, Hard-modding gun attachments (Stocks, Handguards), Ammo
Prapor buys: Grenades, Ammo
Fence buys: Anything (generally you only want to sell irreparable armor/helmets/visors to him.)
In some specific cases, Jaeger also buys a few items for the best value, the only one I know for sure is the Antique Axe melee weapon.
When you unlock the flea market (at level 5) you should generally sell large stacks of ammo on there, because the vendor value is typically much lower. You can also sell low durability armor on there if it can?t vendor to Skier, but fetches a higher price than Fence would pay. Lastly?ALWAYS DOUBLE CHECK VENDOR VALUES BEFORE LISTING ON THE FLEA MARKET.?This is a crucial step.