Looks can be deceiving especially with a subtle morph like a YB. You wouldn't be the first or last person that bought a snake advertised as a YB to find out later that it wasn't. Plenty of normals look like YB but there are differences between the two.
Great example just a few posts down,... sold as but not a YB.
http://ball-pythons.net/forums/showt...a-yellow-belly