You probably will not get much for trade on those receivers, particularly when it comes to the 4000, while the 501 has less storage space than what the 508, 510 and 522 receivers do. A retailer may give you $100-150 for both receivers seeing how he will want to make a profit on the trade-in and does not make much if anything on a new 721 since retailers have to pay close to full retail price for the new receivers.
Try Dishdepot.com website. Talk to Mark and see what he will give you for a trade in for a 721. He offered me 100.00 for my old 508 dvr. He also said he would give me 25.00 for a 301. Either way call the number to talk to him in sales and work a trade in deal . I have done several over the last 2 years and I have upgraded all my old receivers and even traded in my old Directv receivers too.
Yip, I would have to agree. On ebay you would get to keep the money yourself and would probably get more out of the 501. If the retailer buys it he will only buy it for cheap enough to where he can make a profit on it by turning around and selling it to someone else. Dont forget, when doing business on the internet, there is a loss on the shipping as well.
If you are not getting much out of a receiver (such as $25) and then had to pay shipping, you might as well keep it for that, especially a 301. I would buy 301's all day long if I knew I could get them for $25 if I knew they had no balance on them, was in working condition and was transferrable to me because I know of customers that would like to upgrade or need a receiver if their's go bad or need a cheap additional receiver.