Grouper -- I'm a bit puzzled due to the way you have framed your question. Are you asking about purchasing a specific RT vehicle from a specific private owner already under consideration or about any used RT from any private owner (vice any Can-Am dealer, car or motorcycle dealer, etc) that you are searching for? And are you asking about an in-person purchase or about an online purchase?

Your lowest risk is the 2014 and later 1330cc I3 with the paddle-shifted 6-speed (SE6). The power train is pretty sturdy and the "nanny" electronics tend to protect against abuse. A properly-maintained, with service records, high mileage (eg, 50K) RT will continue for another trouble-free 50K miles. A private sale on this forum is lower risk than an private sale on Cycle Trader because members are more likely to take care of their RTs, A private sale with many well-framed, well-lighted photos is lower risk than a few dark smartphone photos because it demonstrates pride and therefore care. And then there are farkles. If you know what you want and the candidate RT already has them that is a big cost saving which somewhat compensates for risk.

If you do your own work, I estimate $500 in parts to establish an end-to-end maintenance baseline (which includes spark plugs). If you don't, then your wallet will decide on your baseline.

So far I've been fortunate with my 2014 RT-S SE6 77K miles that came with a trailer. I didn't meet the original owner but was told the primary use was road trips in the SouthWest which matched its condition. That is, sunny, dry environment; long rides with not much stop-and-go; speeds less than 70mph. So it's pretty much like buying a quality used car.

Best wishes. Post pictures.