There are lots of places that sell pre-cut fabric squares. Could you post a link to the ones on eBay you are looking at? I think it's fine to purchase pre-cut fabric squares.
Pamela beat me to it! After posting, I decided to figure out how much quilt top you could make with 20 5" squares -- not much! It would probably be considerably less expensive to buy yardage and cut it yourself. And I hadn't even thought about postage costs.....