Ron's experience is a scenario everyone should be aware of before purchasing without a title, but with that said, it is very difficult to prove what someone else knew, beyond a reasonable doubt.
You want to avoid buying a stolen bike for ethical reasons, if not legal reasons, and the hassle would suck - but it's not terribly likely that you'd actually be convicted unless there were clear signs that you were intentionally ignorant of the bike's origin.
I I have never seen a ground-off VIN plate at the track. Stolen or not, that's illegal.