Unfortunately, you're going to find a LOT of this sort of thing. The bottom line is, if you want to buy a used Mac, the only place you should do it is from someone you know personally.
You should never, ever, ever buy any computer equipment from eBay or Craigslist. You're just begging for problems. There are a few reputable online resellers, but I wouldn't touch them either. Find someone you know personally. Or go to your company/school, etc. and ask if you can buy one of their older computers.
I dunno, I've purchased numerous used CPUs from eBay, multiple smartphones, a graphics card, and even a Mac mini. It's more about who you're buying it from--sellers with lots of ratings and high (99%+) sales ratings. I got a couple bad cell phones before, and the sellers took them back no problem. Craigslist is far more dicey to me, but that's mainly for personal safety. You don't want someone luring you to their house so you can "check out" their electronics while plugged in. I know the odds are slim that someone will do something, but you only have to be wrong once. Plus some people are just flakey and won't follow through on the deal you agreed to in advance.
Thank you so much to everyone for giving me advice! I've decided to not pursue the computer, I just thought it was too much of a risk. I'll just look elsewhere. Thanks again!
I've also had good luck with Mac of All Trades. That's where I got my cMP. It's not a looker, but it has been very reliable. Look for coupon codes when you buy. I got my 4,1 cMP for $320, with free shipping on a 50lb computer!