I think he is beautiful and I bet once his back sheds, he will be full bodied bright yellow. Yellow is my most favorite color morph, I would snatch him up in a second. Don't let his size deter you, if he is healthy he will be a good pet. Since he is still young, he will grow more, especially if you get him on a proper diet and lighting.
The one thing I would ask for is a full body photo. You want to make sure there is no lower extremity deformities. It would be heart breaking to go get him only to discover he has a twisted leg or bent spine.
Oh, there isn't such a thing as a 11 gallon tank. From the looks of it, that is a 20 gallon long.