Receipts for the rebuild, if he says he did it himself he better be able to show you his case splitter, flywheel puller etc. And he should also have the receipts for the parts that he bought and put into the machine. If theres none of that, stay well away. I've made the mistake of buying the first bike I come across once and it turned out very VERY poorly for me. Like everyone else is saying be very picky, if something seems fishy just take your money and leave.
In regards to its suitability in the desert I agree with Openhiemer. If its just wide open desert you're going to be running and you're still after a two stroke, look at the KTM 200/250/300 exc.