Could be thru an NPC auctioneer, a trading board at the safezones, or some sort of public market to post items for sale.
I believe this will bring depth to the game economy, for players to have an option other than simply selling stuff to the SZ traders at a fixed, and severely unfair pricing. Trading may also bring a new dynamic in the game, by motivating players to pursue certain stuff to gather and sell. Some may specialize in hunting, fishing, crafting, arms dealing, etc..
I suggested a similar idea a long while ago, an auctioneer NPC who would sell player items at the cost of keeping 10% of the revenue generated from each sale.
Player-to-player accept/refuse system