If you have time, I would recommend selling them at eBay.com. You will have to pay commissions and have to ship each of them out to the customers who buy them and perhaps do some customer service but IMO you will make the most money this way.
If you are busy and do not have the time to sell them one by one, there are many websites that buy them back online now. I have always used http://www.EagleSaver.com and have not been disappointed with their service. I think I have used them about 4 or 5 times in the last couple of years. They offered me a free shipping label and there was no cost to use their service. They also offered a quote guarantee and responded to my email in which I had a question promptly.
That being said, I have been disappointed with some of their payouts. Sometimes, they will only pay $.32 for a DVD. To be fair to them, when I looked at what low payout items on EagleSaver.com were selling for on Amazon or eBay, it was usually between $.01 and $.50. In general, it appeared, they were paying between 35% to 60% of the going online selling rate. That seems reasonable to me. For some items, they paid a surprisingly high price. I had the complete DVD set up Columbo and they paid me fifty some dollars as I recall.
If you do not sell them yourself and use another company instead, my best advice is to try the company out with a small sample order and perhaps test the company's customer service. I think there are a certain amount of scam companies out there so you should be careful. I sent in about 20 of my 500 DVD collection for my first order. EagleSaver performed well on this first order so I kept selling back there.
Best of luck!