You propably trying to sell some kind of big guns. Right?
If yes, then go to for example to VC or try to sell other stuff.
Each trader is buying other stuff, ie iguana guy in NCR buy only Golden Nuggets, bartenders buy almost any garbadge(very low quality guns, resources, miscaleus), the guy in the Hub Oldtown and gunrunners buy high tech guns, in necropolis all kind of energy weapon, Jargo in New Atydum all kind of armors, and so on.
You just need to figure out who is buing what.