This is a suggestion for introducing barter between players. Maybe this can work, maybe it won't.
I'm feeling rich right now with more guns than I can carry, more armor than I can wear, plenty of shells, food, water, duplicates of some other items.
I expect that in a year from now I'll be starving. I don't even have enough food and water for a year, and the horde I've amassed in the first few levels ain't gonna last. That's what I imagine is in all our futures.
Not having a "merchant" in game is the correct decision I think. I like the challenge that introduces, and it isn't realistic anyway. But would barter add something to the game?
Barter can be exploited to concentrate powerful items in the hands of a few very active players, and it may ease the challenge of surviving in this game. Worse, game play could become driven by bartering and obtaining items, rather than driven by survival the way it is now at this early stage. It could also ruin one of the present challenges, which is having the right tool or item to solve a particular challenge. Bring in barter, and unless you implement rules about encumbrance, pretty much anyone can always be carrying one of every item in the game, no matter how rare. People can also make alts and pass items between them, cheat in that way.
Still, It could be fun to swap items with other players, and it could add to the feel of a post-apocalyptic economy. Perhaps limit the scale of barter in order to prevent it from getting out of control. Limiting the scale of bartering geographically could also be good, so that you have to really be in range with a player to barter with them (maybe even in the same location? How's that for realism!!).
Lastly, if my suspicion is correct and we're all going to get poorer rather than richer as the game progresses, barter might keep some of us alive a little longer. It could feed into the story too - some items needed to solve challenges could only be obtained by barter, or people in cities might need items that only rural areas have, and so forth.