Well, they don't have any caps. They can pay you in prewar money tho.

Also as a side note, please edit your post and put it in the Fallout 3/New Vegas category, you can use the three dots on the right hand side. Thanks

Thanks

Pretty much what Mand'alor said, they don't have any caps (and neither do you) so you have to barter with them, trading items of the same value as what you want to buy from them. Although in my opinion it's not worth bothering with them, as they rarely have anything besides a handful of consumables, and a single stimpak and/or small energy cell, all of which you can get more of from the vending machines with less hassle.

Personally I love the venders in the Sierra Madre. You can pick up pretty much everything since your inventory is almost empty when you arrive, then trade all of it for pre-war money which weighs nothing. It’s a nice way to bring back a load of caps when you finish the DLC.

I find with New Vegas that caps are no where near as scarce as they are in FO3 though, I tend to have hundreds of thousands left even after buying all the implants by the end.

They take sierra madre chips instead of caps and if you sell stuff they give you pre-war money