Having spent 1.5 years off and on searching for a plane, I had a few solid rules for myself when dealing with the seller:
1. I wouldn't make a trip to see the airplane unless I had the airplane's tail number and at least looked at the last few pages of logs (a seller should be able to scan them in and email them to you).
2. No money was being transacted without a sales contract.
3. There would always be a prebuy and any money I paid as a deposit would be refundable if I didn't like what I saw in a prebuy.
These rules pretty much keep you from dealing with shady dealers. The dealer mentioned in this chain, for instance, never even bothered to return my phone calls, let alone give me a tail number or answer any questions over the phone about the planes he had listed on his web site, so I never had to deal with him. Yes, in theory you might miss out on a "deal" if you follow these rules, but some deals aren't worth having.