Remember that traders were nerfed so equipment is now based on your level with that particular trader.
It is player level * (1+quest_tier_mod). Quest tier mod goes from 0 to 0.3 in increments of 0.05.
Also, they are geared towards specialties more. Trader Hugh will have a more unique stock of weapons compared to the other traders
So your best bet for weapons would be Trader Hugh and working on getting his mission tier up higher for a larger selection. The others will also sell weapons (obviously) but as you can see below, RNG might not be as kind to you if you are working on Jen's missions rather than Hugh's missions
- Trader Jen - 3 to 5 melee and 3 to 5 range weapons - note these can be repeats
- Trader Hugh - 3 to 5 melee and 3 to 5 range weapons - all are suppose to be uniques
Note: Hugh also has another trader group of weapons, similar to Jen's but only 1 to 2 and repeatable.
You should be seeing a minimum of 3 and maximum of 5 at all traders except Hugh (Hugh should be a minimum of 4 and a maximum of 7). It would be interesting to see if this is actually the case. I don't think I found Hugh yet in my current playthrough, but I believe I got Joel and Bob found so I can see what their stocks are right now (one I am working at T3 missions and other other hasn't started yet). I haven't bought any weapons or sold any weapons to them so it should represent their stock available for purchase.
No, stock is based on trader level that can be unique with each person so each inventory can be different between players.