I've put one of my 5970s on BitMit:
https://www.bitmit.net/en/trade/i/7312-sapphire-radeon-hd-5970-4gbThis is a beast but as it is optimized for perf (850MHz and Accelero are standard) it draws too much power to my taste (still mining on it profitably, but it's the least profitable from my rigs). Very good for a gamer (handles itself well vs a 7970 Ghz) and better yet for a gamer who'd want to mine on it too. It seems to overclock well too (at least 900MHz, my power supply and rig configuration limits me here...)
I didn't put it on other sites, as I'd like to make a first sell on BitMit. 5 days left on Bitmit and I'll expand my search for a buyer.
I live in France. It's probably not worth it to send it out of the European Union (high shipping costs and probably import taxes), ask for a quote if you think otherwise.