Run the Serial Number Through a Stolen Bike Register

Do this first, because no mechanical bargain survives the discovery that the bike is stolen. Most serial numbers are engraved under the bottom bracket, so turn the bike over and photograph it. Bike Index is a free nonprofit register, and its search also shows registered bikes with similar serials, which helps when a digit has been misread or deliberately altered. A serial that has been filed off, painted over or doesn't match the paperwork is a reason to leave. A clean result only means nobody reported the bike missing; it doesn't prove the seller owns it.