I had this happen to me. I even had the truck, took it to a mechanic for a test, and we had an agreement so I got a cashiers check and went back to finish the purchase. In the meantime another purchaser who said he had a prior agreement showed. Seller hemmed and hawed and said he couldn't figure out what to do. The ethics were clear to me. He messed things up greatly. It was a dually 4wd with only 100k on motor and supposedly rebuilt getrag.
Long story short, I didn't get the truck. My respect for seller is at zero. I actually believe what happened is I drove it and said it lacks power compared to mine. I then told the guy about a few things that I would do after purchase. He seemed very interested, and I think passed it along to the earlier guy, while I was bringing back my cashier's check. That guy planned to take it to Mexico for a work truck. He then upped his offer realizing how simply these first gens can be tweaked. I was out a lot of time and trouble.
Like I tell my kids though, which one would you rather be: the person that had this done to them, or the type of person that does this? I left and just decided that there would probably be other flaky things on the truck that would cause bad pickup karma down the road. I kept my 1st genner (no problems whatever) and used the money to completely rebuild my wife's jeep cj5 with paint, body work, new parts, and its third replacement motor--one owner jeep. She's much happier. I sure would have liked that dualie with relatively low miles to pull the horse trailer though.