Printer ink is actually the most expensive liquid in the world. It costs more per liter than oil. How we ever arrived at this point, we don't know, but it's true. The cartridges that they come in are valuable, too. Every time you've traded in your printer cartridges, you've given away $5. That's not so much for an individual cartridge, but think about volume.
If you work in an office building, there are probably several printers in there, which require replacing regularly. Offer to dispose of them safely. Even better, approach other office firms and offer a disposal and replacement service. Buy up cartridges, deliver them for a profit, and then make money by selling the old ones back to Cash for Cartridges, or another online site that will buy them. You don't even need to make a large profit selling new ones if you're cashing in on the old ones!