MtGox's secondary market? I remember back in the day when MtGox halted its BTC withdrawals, someone put together a secondary market exchange for trading MtGoxBTC for actual BTC. You could deposit MtGox BTC to it since the exchange still allowed you to move coins between accounts even though you couldn't withdraw the coins into external addresses. I tried searching for it, but the amount of articles on MtGox collapsing and so on seems to have buried that old titbit of information.

Does anyone have any information about MtGox's secondary market, what it was, how long did it operate, what were the prices and so on?
It looks like Bitcoin Builder was the website. While the main focus of their site has shifted, the front page still refers to goxbtc.