URLs for Used Products - What to do?

Taking this to the creative SEO hive. Anyone have any experience with e-commerce, specifically sites that sell used products?

I have a client that has singular, one time use URLs for all their used products. Once those sell, they go to out of stock and fall off on-site search. However, the link remains live forever. What do other SEOers suggest to ecommerce clients with unique previously used inventory?