When you list an item for sale from your wallet, we have to scan the smart contract and make sure it will work on our marketplace.
If you see a message that says:
"invalid smart contract" or "not a valid smart contract": you are using an address that DOESN'T belong to a smart contract. Double check your address is correct by looking it up on viewblock.
This item cannot be resold: The creator of this item doesn't want people to sell it.
Token ID doesn't exist: You entered a wrong token ID that doesn't exist on the smart contract.