I once tried to take over someones spot on a GB.. I was asking for their invoice / what options they had. They replied basically asking me if I had any artisans he could get for the spot (on top of his original invoice of course).
I was just like, WTF. It also bugs me when people get some relatively exclusive custom and it's immediately up on MechMarket/Classifieds w/ a huge markup.
(it's totally understandable when a GB takes longer than anticipated and you no longer want/need it -- be one of the good guys and sell it at cost!)