I wouldn't automatically assume that an unbelievable deal mean the item is stolen, although I concede that it is a concern. After I bought the 7D/10-22, my wife brought up her suspicions that it may be stolen. I checked a stolen camera registry and it wasn't there. Also I have the seller's phone number and name, so that assuages my concerns.
Sometimes people are holding onto stuff that they need to sell off quickly and price it accordingly.
Some deals are good but some are too good to be true. Like for example, refurb 24-105 and someone is selling it for $650 since it was on sale a few weeks ago is believable.
A deal on CL listing the same thing but "NEW" is questionable.