have a look round CEX, i picked up a nice 6950 the other day in exchange for a phone and some ps3 games, its happy chugging away at 380Mh/s using bitminter
7870xt or 7870 tahiti le are good value, and usually on offer on Aria and are good for 500Mh/s and come with free Steam games you can sell on

i guess having a bunch of spare pcs/servers lying around in my house helped me kick things off.