There are many websites where collectors can sell sports cards for cash. Online retailers like Dave & Adam's will pay cash for sports card collections. Other sites such as Sports Card DataBase offer users the opportunity to auction off their cards to other users.
There are many online stores specifically aimed at buying and re-selling sports cards. Dave & Adam's offers to appraise the cards or collection and inform the seller of the cards' value, free of charge. The webpage offers many categories to choose from to specify what type of collectibles are being sold. If a price and sale are agreed upon, the website buyers are able to pay the seller in cash or via PayPal, among other methods.
While Dave & Adams offers individual appraisals and direct buying, Sports Card Database is an auction-based collectors' market which offers appraisals, a current database of available cards and an online community where registered users can put their cards up for auction and search for sales by specific users.
Ebay may be a good place to sell sports card collections, though in this case the potential vendor will have to do their own research to find a fair price for the collection. Users on Ebay can browse through many different collections for sale, so there is no guarantee that a collection will be viewed or purchased.