Obvisously, magento.stackexchange.com would be ideal, but this site might end up being "big" enough that eBay, Inc. might wish to exercise their trademark rights.
I say go with the obvious. I.e. put it up on magento.stackexchange.com. If eBay ever did try and take it down, they'd be kicking themselves and frankly I don't think they would ever try.
The only other alternative I can think of would be mage.stackexchange.com, but then we might have wizards getting involved...but that would perhaps be a good thing.
"mage" is not bad, but I also would vote for most obvious - magento.stackexchange.com
mageoverflow.com is still available.
What about magento-answers.stackexchange.com? Just kiddin' ;-)