Even better surely is to just tell the printer to report the toner as empty after say 30% use (like manufacturers do with inkjet carts!). Then when the customer sends the toner cart in you can recycle the toner (or just top it up and reset the cart or whatever) and sell it back to the customer ... instant sales increase.

Of course after a little while the customers going to notice a problem. So then you can sell them a fix (patched firmware) or a new printer. Play it right and you get to run the same scam again then ...