I'm lvl 10 and just got to a town somewhere east of good springs () and I think I've run out of places to sell items. I've been around the bottom of the map, so those places are mostly dry. I have lots of gear that I'd like to sell, but apparently I've sold too much to the people i've met and I can't find anywhere to dump some of this loot.

Do the merchants ever gain money back, or do I have to keep exploring to find more?

Yes merchants reset after a few days.

Also in Novac a guy has a shop in the Dinosaur (in case ya didn't know )