Can't tell much from the pics. A link to the original would be better--the one posted is very small. Yours looks more like the one I threw away than the one I paid for. As I said, I just paid $500 for one in nicer shape and with both control arms in place in front and a perfect hitch in back. I will say the surface rust on mine--there was less of it--cleaned up very nicely and looks perfect after POR15-ing the whole thing.
The key thing you want to check is how solid it is on the bottom of the rail inside the big control arm bracket in the back. This is hard to check with the frame on the ground, but it is worth jacking it up a foot or so and then banging right there with a hammer and big screwdriver or rod up through the center of the bracket. They rust from the inside out, if mine is typical, so just a look can be deceiving. I would not pay a dime without doing this test. Mine was on the body still when I bought it, on blocks with no diff, so it was easy for me to do this.
My frame had sat there since 2011, so they don't move that fast, at least in NJ. Offer him 5 bills cash and watch him take it.