Car plates are different, I dont think the MTO cares about getting those back if you cancel your sticker where as they care about getting your bike plate back. I still have my old plates from my old car in my garage, I guess I could always switch back and have Service Ontario re-issue me new stickers without the need of buying a new plate.
They would care if those plates suddenly found their way onto a different unlicensed vehicle, as they're still attached to you. They would make you care too, if police and MTO caught up with who was using them.
Yes, the posted kijiji request is a scam and I pity anyone who falls for it.