When I bought my 01 S used from an out of state seller, he gave me 3 days of ownership to change my mind, and put it in writing. I could do any level of "PPI" that I wanted during this period, and he would take it back for a full refund with no questions asked if I wanted. But if I damaged anything by wreck, destructive inspection, whatever, it would be on me. This seemed fair.
The 2 day drive home, after a PPI at the local dealer, was a good shakedown. I found evidence of a prior wreck mentioned in the presale discussion, and replaced a MAF; neither was sufficient to make me want to back out after just a little seat time.