There are basically two angles to this, I'll just answer both and be done with it:
If the OP is the parent considering the purchase of a second-hand swing...DON'T - isn't your child worth the extra few dollars you'll spend to buy new?
If the OP is the person considering selling a second-hand swing...theoretically, you could end up on the wrong end of a lawsuit for various reasons related to this matter. Why not simply donate it to Goodwill, or some other charity?