First time I went to Gun'Runners I was below lvl 10. Now Im lvl 20 and the items on sale are still the same! No sniper rifles or any good weapons there. I have the problem with all the merchants they arent getting any new ammo (running out of magnum) or doctors backs or anything. Im pretty sure merchants should be getting new stuff once in a while. Im on PC using only Arwen realism mod. Anything I can do?

How is your time-setting with Arwen Realism?
The traders are supposed to restock every three game-days... i found that the default setting of Arwen does take a hell of a lot of gaming to get those days over. So maybe you want to check.
Also ... at least at the Gunrunners you might try saving in front of the shop and leaving the game. Then starting again the shop should be restocked (worked for me)

Thanks it was on default so I changed it and after waiting it restocked.