Looting and Shopping:
Deadside loot is pretty easy.

An item you buy for the X amount can only be sold for 20% of its purchase value. Some items cannot be sold as follows:

Certain jackets and clothing
Ammunition that is not stacked by x100
Rags and sterilized rags
etc.
Here's Zelerity's guide to Safe Zone Trader pricing!

* Note a few things that say nothing about the sale. Sometimes it needs to be updated, but sometimes an item cannot be sold. All weapons in the game can be sold.