But against the billions of people who surf EBay every day the chance of being outbid by another forum member is pretty small. I can remember twice when two members were looking at the same item.
I can actually think of several instances where I was bidding on items that ended up being won by fellow board members. And I didn't even know I was bidding against them until I saw them post about their new acquisitions later. So it may be more common than any of us would think.

There have also been cases where I was bidding on an item, only to see it posted on the forum by someone else, and suddenly the price skyrocketed. I can't begrudge the winner of his victory, because after all, what it simply came down to was that he was willing to pay more for it than me. But still, it kinda burns when that happens solely due to a public 'outing' of the auction on the forum.