Primarily community driven to raise red flags about stolen bikes. That said, it's pretty easy to look at an advertisement for a bike and know if the seller actually knows anything about it.
Many bikes photographed in the same spot. Poor-quality photographs (blurry, etc.) that make it difficult to pick out distinguishing marks. Very little detail about the bike's components.
None of these are proof of a stolen bike, but each is a useful flag, an indicator that the ad is worth further scrutiny, and flagging could help people looking for their bike.