Peerflix.com is a good site to trade your dvds you don't want anymore for ones you do. It operates kind of like the rental places on line, except you don't return the movies. you send them off to the next person who requests it. Sometimes it takes a long time to get any of the movies on your list, but it does kind of give you an idea by telling you for each movie how many turns you have to wait. One drawback is that you don't get the box or the inserts for any of the movies, so if you get a movie you want to keep (which you can do) you will only have the disk.
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