I was looking online for a new 512 stick and realized that they are expensive for what you get. Is buying on ebay a bad thing if I wanted RAM? Can I find a reliable seller of computer parts on the world's largest swap meet?
Macs are very particular about memory. A module that works just fine in a PC, may not work reliably in a Mac.
I've also found that the installation routines in OS X are even more particular about memory than OS X is once it's installed.
Memory that will pass Apple's hardware test, and work fine most of the time in OS X, will still cause OS X to fail installation.
So, you really should buy your memory from a place that knows Macs, and will certify that it will work in your specific machine.
For that purpose, I have found the best guarantee and customer experience from Other World Computing. I've used their memory in around a dozen systems, and have yet to find a single problem.
http://www.macsales.com
They know their stuff, they are great to work with both before and after the sale, and I've just been a very happy customer.
The extra you pay from a place that knows their stuff can pay-off big when you compare it against your options when something goes wrong with an e-bay auction.
Personally, I would never buy memory from an auction (or even second-hand), unless it was priced so cheap that I could throw it away without feeling upset about it.
Memory is one of those things that can come in varying qualities. And, even if there's nothing wrong with it, it may still not get along with your Mac. And, beyond that, it is (like any other electronic component) easily damaged by improper handling (such as static discharge). Don't buy your memory second-hand unless you get it cheap enough that you consider it disposable.
If you do buy it second-hand, make sure you are absolutely confident that the seller will guarantee it to work when it arrives and refund your money if it doesn't. And, don't pay anywhere near retail price for it.