570 vs 660

Someone is offering me their slightly used gtx 570 2.5 gb for $100 but I already have a gtx 660 2gb is it worth the money? should I buy it?

Easy choice, keep the 660

If I didn't already have the 660 would you recommend the 570?

The 570 is a capable card; but I wouldn't buy it, personally. I would look for something newer.

The 660 is the 600 series equivalent of the 570. Just as the 770 is the 700 series equivalent of the 680. The relative performance of a GPU moves down the model number hierarchy with each and every updated series.