I've always owned a Mac. Had a 5500 Power PC in the 90's, then a Flower Power G3, and now have a MacBook (black) with a core duo processor.

I bought off of Ebay, but I also did a ton of research and also talked with the seller. You get what you pay for on ebay. If you want a reliable laptop, with a good amount of apps on it, as well as a large hard drive and memory to spare, you're not going to get it for $500. I'd be wary if I were you. Shop around and ask the seller questions about the laptop/desktop.