Sebrights only come in Golden-Laced and Silver-Laced, not Buff as they always have to be laced. They also have an upright breast, vertical tail and slate legs. Your bird is not even remotely close to a Sebright.
I think that is merely something that people have bred and dubbed a "Buff Sebright." All Sebrights are supposed to be laced, and lacing is one of the most important parts of a Sebright.
Sebrights are laced and have a more vertical body shape, especially in the tail, and they do not have pink legs. The OP's bird cannot be a Sebright due to that.