In some ways, Applecare, like any extended warranty for an appliance, car, or other product, is always a bet against yourself (or your product). I've never purchased Applecare, and never really needed it. There are plenty of folks who have purchased and needed Applecare, so statistically my experience shouldn't tell you anything. A good rule of thumb though, is that if an electronic device is going to fail, it will fail well within the first year - more likely in the first few days/weeks of operation. If the eBay rMBP has survived a 18 months of trouble-free operation, it's doubtful any components are on the verge of failure.
In your situation though, where it's 2nd-hand and you've experienced device failure in the past, it can be worth the peace of mind alone.
What I like about the retina mbps (or any device with non-moving parts) is that there's less mechanics to break down. In my old PCs the hard drives were almost always the first to go. Nothing else "failed." My brother's optical drive failed in his iMac.
I'm assuming you're getting a good price on the rMBP on eBay (vs. purchasing refurbished from authorized reseller) - if you trust that it's in "mint" operational condition, Applecare might be worth it for you, to give you "new-device" protection. Another thought is to find out the cost of an average out-of-warranty repair for a rmbp (I don't know what that is, but it's probably more than Applecare). If you're willing to eat that over the cost of Applecare, save your money.
In any other case when buying new or certified refurbished, my opinion and experience is that not getting Applecare is *not* a bad idea. Plus, American Express has built in extended warranty.
Sorry if I haven't given you a black and white answer, I just wanted to give you something more than "of course get it" or "it's a waste of money."
Either way, just enjoy your new notebook.