I'm only a very occasional EBay user, but I'd assume that since the auction ended early, at the "buy it now" price, that $100 for the Rivers set was regarded as a fair price. Either that, or the bidder wanted it REALLY bad, and didn't want to compete.
If you decide to list that one on EBay, you might want to try it without a "buy it now" price, with a minimum reserve of $100.
Another alternative (which I may try on a few of my own Mosaics) is to list it on this board first, at a "reasonable" price. I'm also struggling with how to determine that price to make it fair to both buyer and seller.
Oh, and good luck on your project!