If you own an NFT, you can see the Sell button on the NFT page.
Click Sell button, it may pop up a modal, saying You need to deposit NFT before selling it.
So continue to deposit.
Scan the qrcode with Mixin Messenger to deposit NFT.
Click Sell button again.
Then fill the form and click Sell.
Pay 0.00000001 XIN to Trident MTG to invoke the request.
And then you should receive the success notification.