Yes for items like bikes meet face to face.
Also check their feedback
If there are loads of positives for buying small items in a very short space of time could be suspect.
With bikes check the write up against the specs from manufactures website or well known bike shop. If somthing does not look right, ask questions of the seller. If you are not happy with the reply say away.
Also remember this IF IT LOOKS TO GOOD TO BE TRUE IT IS.
We have been selling stuff on e-bay for a few years and have pos rating of over 600.
Only buy if you are 110% positive that it hasn't been stolen. The seller should be able to tell you a story about why he had the bike and why he is now selling it, including showing you receipts and explaining how he got scrapes on it. Your own experience will also tell you whether he's a cyclist or a scrote. If he's "selling it for a friend, don't really know much about it mate" then assume the worst and walk away.
Buying stolen bikes creates a demand for them which leads to...more stolen bikes.
I advise reading feedback carefully. Look specifically for -ve or neutral votes - not just the overall percentage (most major sellers will have some negative feedback). Then check some of the feedback the seller has left buyers, if there have been problems do they appear to be interested in resolving them or just giving retaliatory negs in return?
This online tool makes it easy to search out neutral and negative reviews.