There are two types of Trading you can do in this game, with NPC and with other players when playing on-line.

Each survivor camp can trade items and each one will trade different items.

Some of the items traded can be traded at other survivor camps and some of these chained trades or routes are discussed here.

There is a Google doc page that has these routes and calculates how much each trade will bring you and also calculate the weight of the items so you know if you can carry it all.

Trade route sample

Trade Routes

Below is a good source for location and what trades can be made at each camp

Survivor camp trade list 451-01 with obvious trade route tips

You can also look at Trade routes organized as a table in this page