Breaking in a Brooks isn't a myth ... when a Brooks is broken in, it looks quite different from when it's new. The saddle in the foreground of this photo is new. The other saddle is well broken in (and, incidentally, very comfortable).
In your case ... who knows. If the person hasn't ridden the saddle much and it isn't really broken in yet (i.e. it still looks like the saddle in the foreground of the photo above), then you're starting from scratch like you would with a new one. Or perhaps that person might have sitbones similar to yours and you'll only have to ride it a little while before it feels right to you.
If you had a picture of the saddle, that would help with your decision.