Have they offered you any photos of the snake? If they are willing to send photos, request something specific such as right side, left side, belly, as well as an overhead shot of the snake laying next to something like a ruler or $1 bill for size reference. If they send you whatever photos that you request, then you know they actually set the shots up and didn't go ripping some random photos from somewhere on the internet.

If they say that they can't send photos, or they have no access to any kind of camera, etc, proceed with caution...