SoS's tips are a good way to spot an iffy ad fairly quickly.
If you've got as far as asking questions of the seller, you'll find that a genuine seller will be happy to answer questions and will be quite knowledgeable about the bike.
Examples:
"I'm 5'9" and it fits me perfectly, prefer a longer stem on a 54"
"Chain was changed 6 months ago, I ride 100 miles a week"
"Had it since new (2006), upgraded wheels to Fulcrum Racing 3s in 2008, got all the receipts"
"It's marked with a postcode/Datatag"
Of course, none of that is proof (apart from the receipts) but gives an indication of whether the seller is genuine.
Then there's the old favourite from dodgy car salesmen pretending to be private sellers: When you ring up, ask if "the bike" is still for sale. If they say "which one?" then they could be someone who's regularly flogging nicked bikes.