If you totally trust the seller and you are sure that they are completely fine, you go better with the 6950, it's a good deal for just only $150 and a great upgrade from a 6870. But if not, get the one sunnk said about a 6870 which will be enough for 1080p and good for you if you don't trust these used items.
280 is actually not good, it is slower than both 6870 or 6950. About between 6790 and 6850 probably. It's also lacking of today's technology like OpenGL 4.0 or DirectX 11. The 6950/6870 will also support eyefinity. In my opinion it is not worth it to get older card just for a not much price decrease.
6870 is better because it's new and you know, new is better. It's selling around $150-$170 which is near to the used 6950 that you said before about. The performance of 6870 will probably enough for most games in 1080p Medium to High, but compared to 6950 they are definitely slower. The 6950 will also be more future proof.
So overall, if you trust the 6950, there is no reason to avoid it.