It's never been confirmed that you can buy t3 off the BM. I know there was the picture etc, but still...
Anyway, let's say it's possible. You're going to need tons of golds, by tons I'm talking about 200k+. Each server should have atleast few super rich people who will outbid you easily and push prices ridiculously high. If I'm not mistaken 10k is the starting price for t3 items. I won't even bother trying. And there's no guarantee you'll even see the item you want, BM updates only once a week I think.