as of feb. this year staples will give you $3 store credit for ANY cartridge turned in. and it doesn't matter what brand or even if they sell it. i've taken toner, ink carts from a wide range of sources.
you can turn in 10 per month for the credit. beyond that they'll take them but you only get credit for the first 10.
initially you sign up for staples rewards, they basically enter your card number when you walk in and print you a receipt showing how many you turned in.
you get a store credit check mailed to you each month - that's valid for 2 months.
you get the store credit and they stay out of the landfill.
they possibly could take more if you signed up as a business - not sure about that part.