Yes. Regardless of price, if he is selling it for less and you know it works 100% then it is without a doubt worth it. The 6970 is in a different class than the 550ti. The 550ti is more of a lower mid-range card geared toward budget gamers. The 6970 is last gen flagship card of AMD if you don't count the 2 processor 6990. Check out the comparison below. The 6970 is nearly 50% faster in everything. If you can get it for under $100 it is a steal.
Also, check the other comparison. The GTX 670, a $400 card, is only 10-15% faster than the 6970. The only downfall is that it is PCI-e 1.0. But even with PCI-e 1.0, it falls somewhere between a GTX 660ti and a GTX 670 in terms of performance.
The 6970 is still being sold for $250 new some places.
http://www.hwcompare.com/9707/geforce-gtx-550-ti-vs-radeon-hd-6970/
Thanks]http://www.hwcompare.com/12546/geforce-gtx-670-vs-radeon-hd-6970/