I'm at camp McCarren I just turned him in. Of course, he's gone now and there is no new NPC in the supply shack to sell/buy from.

Will another NPC ever replace him or did I just permanently lose out on a vendor with 8000 caps and a ton of ammo for my preferred weapon type?

Yup, you're screwed.

what is the deal with him anyways. i had a feeling he was up to no good when he need extra weapons and stimpacks and then told me he had one more job and to return after a day. so if anybody would like to enlighten me as to what happens (i wont be able to get on until late tomorrow).........

If the guy had a bunch of useful stuff to you, why would you turn him in?