I'd say around 5-10% of all market trades occur for the sole reason of obtaining an item that user wants to use. Most people on the market are trying to profit from their sales and purchases one way or another. By banning TC selling you're basically killing the whole market, probably remaining duellers as well.

There are risks and there are many loopholes to sell TC without being verified, that's just inevitable due to nature of these features. That said, it's way better solution for everyone involved than straight up banning sales.