I've bought and sold plenty over the years... generally I deal with people with some track record and even on the very odd occasion something does go a little wrong, things generally get sorted out amicably.
An early one was a carbon Raleigh I bought for a mate as a present together with this wife for his 40th.... was sold as 'one careful lady owner' - only to turn out to have been Mannie Heymans race bike. Seller actually had also been duped a bit, as he was getting the parts, but ended up with him paying for a full respray of the bike and it served it's owner well for a good decade in the end.
I'd say the most common mistake people make is grabbing a 'bargain' second hand groupset... 8 out of 10 times it's getting upgraded for a reason. Unless it really does have low mileage - treat it as a consumable. Shifters and RD's it's often really tricky to tell how much use they've had.
Also sold a set of wheels once that I got on a bike I bought and stripped - kept the frame and flogged the rest... the wheels were very slightly out of tune, but sold them stating as much, only to find out they couldn't be tuned as the spokes had fused to the rims. Refunded the buyer in full in the end, but it was a bit of a time waste for everyone.
Like most things in life, communicate, be honest and things will generally get resolved.
And if you do encounter an unremorseful douche in the classifieds, please for the sake of everyone, drop them a rating
