Why? If I check that the mint is available, and then send you coins, and you say “I can’t redeem these, therefore I won’t give you eggs” then I just redeem them myself and cancel the transaction.

If the mint is not available to him, the seller simply need not hand over the goods. 

And btw this is why I think it’s totally probable for most ecash to settle via lightning to the receiver’s preferred lightning provider (whether self-hosted, blind mint, or custodian) before goods are released. In this case, ecash issuance is just the method to bring a great degree of privacy to what would otherwise be a custodial relationship