I've run about 25-30 box breaks on eBay and never once received a complaint. I have just shy of 1,000 positive feedback.
There's a few keys I've learned to make sure you can't be scammed.
When the auction ends I show that the last team ended, who won it, final price etc. As well as the current date and time (I use my smart phone). Also when breaking I say who won the card. For example if a nice Ryan Miller is pulled I'll say a hit for (enter member ID), not just the buffalo sabres.
And yes, entering the serial number in the description is a must! And obviously show the number when you break.
I've sent out some nice cards too, a couple Silhouettes from Crown won for like $3-$8, a LaFleur /25 from SPA for like $12?
eBay has since cracked down on box breaks but it was fun when it lasted. The key is finding a trustworthy seller, it's a shame some people can ruin it.