We know what happens to cartridges sold/exchanged to third-party vendors: They refill and sell them to you. There are many third-party cartridge vendors and recycling services; a casual Google search provides thousands of choices. Some vendors will buy them back from you for about 50 cents to $1.50; the lower price is for previously refilled cartridges. Not much of an offset to the price of a new printer cartridge.
The following examples of other third-party programs are representative, but not necessarily recommended.
Third-Party Mail-in Exchange
Pacific Ink offers a deal in which you can save $2–$3 when you buy a prepaid mailer over the usual price of a refilled cartridge from them.
Cartridge World offers a carry-in refill service at quite a few franchise locations all over the world. You can find out whether one is in your area by going to their web site. I tried out their service with a black ink cartridge. I don't know how well their photo printing works; the IP3000 I own uses black for text-printing only and the CMY cartridges for photo/high-resolution printing.
I took the cartridge to a Cartridge World location and mentioned that I was doing an article about refilling cartridges. The manager let me into the back room to watch. He drilled a hole in the cartridge, injected ink, used a hot-glue gun to fill the hole, and taped over it with plastic tape. No mess, and I got the cartridge back in a few minutes ready to go. It printed without any trouble. For larger orders, they pick up/deliver refilled cartridges. Cartridge refill prices are typical, about half the OEM cartridge cost.
The manager said that the parent company reverse-engineers the inks sold by the printer vendor and provides training on how to refill cartridges. They also have one or more vendor-specific custom machines for refilling cartridges, which I didn't get to see in use because they refill Canon cartridges manually.
I had some problems with the black ink smearing when directly adjacent to color ink after I first started using it, but they disappeared shortly afterward. Figure 1 shows a sample of text made right after I installed the cartridge refilled by Cartridge World; Figure 2 shows an image printed at the same time. Figure 3 shows how the text looked a few days later (compare it to Figure 1), and Figure 4 shows an image printed at the same time as the text in Figure 3 (compare it to Figure 2).
Figure 1 Cartridge World text out of the box.
Figure 2 Cartridge World image out of the box.
Figure 3 Cartridge World text a few days later.
Figure 4 Cartridge World image a few days later.
Despite the problem I had, I still think that most readers will find this vendor worth a try at least once. It's an inexpensive experiment, given that an ink cartridge once run dry is a complete or almost complete loss. Cartridge World warrants their cartridges against problems or printer damage. If this technique works for you, it will save you money, and if you have occasion to bring in cartridges by the bucket, you're going to save significant money.
Other Carry-in Refill Shops
Cartridge World is the only major service I could find that's available either at the international level or at the U.S. national level, although it's possible that some white-box or refilled cartridge vendors offer this service to customers locally.