Depending on how much you think your knees cost....

People throw away old skis because their camber is gone (i.e. when you put skis on a flat surface, you don't see much curve under the skis).

Second hand/used skis may also have problem with binding. So if you buy or pick up used, be sure to have a ski shop technician check the skis.

Talking about throwing away skis, I threw out my last pair near Vancouver at the border because I bought my last pair of skis in Vancouver when the exchange was still good....