Howdy, Brooks.
As several others have pointed out, it's a fair-weather homebuilt camper. Expect to do quite a bit of work to get it water tight. The frame and axle look to be strong and in good shape. I noticed there doesn't seem to be any undercoating and the underside looks pretty clean. That suggests to me that the Kahleefoornyah owner didn't take it out much. Because it leaks? It does rain in CA, just not in the summer time.
I'd say offer $1,000 (or less) and go to $1,500 tops. That's assuming you want the project. If Neighbor won't come down in price, be nice, but walk away. My guess is he's like to have the extra room in his garage more than the extra bucks.
Good Luck!