If I were looking for a van, I wouldn't stay clear. I'd take the guy to AAA with me to complete the transaction or no deal. Just because he didn't register in his name isn't necessarily cause for alarm. If I were trying to flip it for whatever reason, I wouldn't register it either.
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2005 E350EB, 6.0, UJOR 6" lift, 35" tires and more.
I have risked this on older, cheaper projects, none over about 5 grand. 20 grand seems a lot more scary. I would push the guy on talking to the owner on the title. If he balks at that then I would run. If he doesn't object then he's probably honest. As for trying to skip any taxes when possible, I agree with the guy.
Since he is not a dealer he is not simply skipping taxes passing them to the latest buyer....he is cheating and stealing money that he owes to the state when he took over ownership/control of the van. Stipulating "Cash only" is perhaps because he is insistent on covering his tracks from getting caught.
To each his own choice..we were raised to not lie, cheat or steal or tolerate those that do.