fairly sure the Arms Dealer only sells stuff to you and not buy stuff from you.

Ok, I did found an anwser to my question, he can buy, but only stuf you didn't unlock in infinite quantity, that why I couldn't sold him my stuf.

Love the mod is amazing, But i find the I'm not able to sell certain weapons to the arms dealer after looting all the bodies. They dont even show available. I'm using the supported RHS canada mod straight from their website.

I think what you can sell is pre-coded, so not everything can be sold to dealer. And there's limited amount that dealer can buy, so if you sold a lot of same weapons to him then one day you can not sell that weapon anymore. Not sure the exact number but maybe 255 or so.