1st: talk to your Instructor so you know what you are looking for. You might fnd it better to get a decent Beginner's grade set NEW, sized to fit you. It may not last as long, or it may last better, and you can get a new set for under that from several different vendors online.
2nd: If you decide you're still going with a used set, then do some research. Learn the terminology, the right sizing, what makes for good quality (materials, stitching), and where to watch for wear-and-tear (inside the palm of the gloves, padding).
3rd. Check e-Bay. And KEEP WATCHING!!!!
Use keywords like kendo armor, kumdo armor, bogu, and hogu.
I bought two complete sets, parts of a child's set, and four shinais for $400. Between all the parts, I ended up with one and a half decent sets of armor, and used shinai for any beginning student to use who visits our class.
Happy hunting!