The trading in-game is broken in terms of what you can sell to merchants. (no shit)
I suggest we add a mechanic where the merchant is more picky about what items the trader is interested in, what he/she doesn’t need.
No one is ever going to buy 10 back-up beepers from you just for your own benefit.
If trade is being improved anyways, it might also be a good idea to vary the trader’s inventory from time to time. Like they’ve been trading with other people in the meantime, and have bought and sold some stuff. At least for the static NPCs whose dialogue implies they’ve got other people they trade with.
The game UnderRail did a good job with trading. Traders would be searching for specific categories of equipment, in varying amounts.
Sometimes a trader would always buy something from you, or would buy only a few items, and sometimes they would be after a specific item and pay really good money for it.