The flip side of that coin is that if you do not know the individual well, they could be selling you their problems and simply not telling the truth about where they got it, how long they have had it, etc. We tell our customers that we will support the new owner if they sell their tractor (if we wouldn't we are severly limiting our customers' options) and we offer to confirm to their potential buyer that the tractor came from us. Ask the individual where they bought it....ask to see an invoice, or a copy of the invoice with the price blacked out if they choose. Or take the serial numbers (engine and chassis) and ask them if you can contact the dealer who sold it to them to confirm that the source is that dealer. If they get vague on you, I would steer clear.
As always, your best guarantee of satisfaction with your tractor ownership is a reputable dealer. If you are unsure of what you are getting and you order parts to find they just don't fit or work, you're generally just stuck with them. On the extreme bad end of the scale we (and Len Sheaffer ran into this several times) have known of a seller that was marketing repainted and rebadged Shibauras as Yanmars. Those folks got some very wrong parts when they ordered.
A few years ago we learned of a "dealer" in one of the Carolinas who was selling recons and telling buyers their tractors came from us. After the second call we contacted the dealer and advised them that we were sending these customers confirmation that the dealer was a liar, and asking them to tell all their friends and neighbors. That apparently stopped quickly. BTW, I am not aware of that dealer being in business currently, so please don't make any assumptions about any current dealer.