I purchased my current Mid-2014 maxed out rMBP brand new on eBay and it even came with apple care without my knowing for $2000 about 6 months ago. I've also purchased two previous rMBP's before my current one on eBay at incredible prices, all brand new, and received no problems at all. I don't understand what the risk is. Just purchase from reputable sellers. And even if something goes wrong, eBay will fix the problem. It's not like you're going to end up with nothing - you always get your money back if there is a problem.
Honestly, I don't understand why more people don't purchase macs on eBay. You can really get some awesome deals that beat any other seller online.