Has anyone come across a government website that allows you to run checks on vehicle number-plates (to check for unpaid 'tenencia' and/or unpaid fines)?
You need to ask the seller for all of their tenencia payment receipts. If they don't have them or refuse to provide them, the tenencia is unpaid and it could be for a number of years.
Just last year, I made the mistake of buying a second-hand scooter (at a tianguis, so I got what I deserved) without receipts, only to discover that there were 3 yrs worth of unpaid tenencia. It still hurts thinking about it!