No surprise. I expect them to only continue higher as the 2012 quad will be the fastest mac mini for possibly years to come. Bonus that you can add RAM yourself and room for two 2.5" ssds.
I listed a new, sealed one the other week. Set it up as an Auction with a Reserve Price. I set the min. bid amount to $620. I also offered a Buy It Now option ($975). Within 4 hours, I received an email that my 2.3 quad sold!
eBay fees are 10% of final selling price, which includes shipping charges. Then there's additional $10 - 20 expenses (if you didn't add these to "handling" fees) to cover insurance and extra shipping options like Signature Required. On top of that, PayPal takes 2.9% of the total paid by the buyer.
While it sounds lucrative, the fees cut into your profit. I would highly recommend a Reserve Price to not incur a loss (note: eBay charges 1% of the Reserve Price..and it's charged even if the item is NOT sold - e.g. you decide to close the listing early.