As Eagle said, our trucks tend to rot from the inside-out, meaning that if you can see rust it's probably 10 times worse than it looks. make extra double sure that it's not doing that. there's a big difference between a rust-free rig that's derivable and a needs-the-same-amount-of-steel-surgery-as-if-it-had-actual-holes truck that's still in pieces.
I smell a scam.