It's probably OK, but for GPUs, with these ebay auctions with no "Buy it Now" option, you tend to mostly wait a long time and then the price is almost what it would be if you bought it new. I'd probably wait for a sale, to be honest.
What's more, rather than crossfire that card, I'd probably just wait a few months, sell your existing one, and get a better single card ... GTX 780 Ti, 980, etc. Much better off in the long run that way.