Before you are able to sell your .zil domain on Mintable, you must first Upload your .zil Keystore to ZilPay.
Go to Mintable and Login or Create an Account.
After logging in, click Sell.
Then click The NFT is already in my wallet.
Click Unstoppable Domains (you guessed it!).
ZilPay will open. Click Sign.
Type in the domain name, fill in the rest of the details (price, description, etc), then click List This Item.
Your domain is now for sale!