I discovered that I am vulnerable to price speculation selling these gift certificates on Ebay, so I had to cancel my $100 auction early.

Assuming the e-check clears, I will still pay the 14.29 bitcoins to the winner of my earlier auction.

The problem is that the winning bidder can pay by e-check, which takes a week to clear. The winning bidder can cancel that e-check at any time. If the price of bitcoins doubles, they let the e-check clear, and I send them the gift certificate that they bought at a much lower price than it is currently worth. If they don't like the price in one week, they cancel the e-check before it clears, and I have to re-list the gift certificate. Ebay rules won't let me exclude e-checks.

I've learned an important lesson, and I'm changing my strategy. From now on, I will be selling gift certificates good for MtGox USD on ebay instead of bitcoins. Since the price for them doesn't vary wildly, there isn't a vulnerability to price speculation.