I purchased a one year used P3 in 2016 off of the classifieds. I think the best you can do is search through the users post history to try and get some sort of idea who you're working with. Ask for pictures, and lots of them. If there is something that appears off in one of the photos then ask for a closer view of said area. Lastly, make it clear that if you/a mechanic notices defects on arrival then you'd expect to give the bike back in exchange for funds paid, less shipping - not sure if any of that will hold up in court, but if nothing else it's a documented agreement and you can use it to get the ST pitchforks out - which may help.

All that said, I had a good experience and would do it again. I actually posted a request for a road bike on the classifieds in September prior to purchasing from a different outlet. Like you, I think most people on here are honest and not trying to pull one on anyone.

EDIT: I wouldn't expect a seller to split costs with me if I wanted to verify the bike is in working condition. That'd be a write off I'd expect to deal with on my own - if the seller in good faith believes they're selling a good product, further inspection is on the buyer.