We can allow Alice to include a Monero reserve proof in her quote which she sends to Bob. This way Bob can verify if Alice does possess the funds she offers for sale before initiating a swap with her.

This will provide some mitigation against potential spammers. Bob will check the validity of the reserve proof using check_reserve_proof before initiating the swap setup. Alice cannot offer to sell more Monero than she can prove she owns.

As long as his Monero daemon is fully synced, Bob can check the validity of a reserve proof even if his restore_height is higher than the height of the outputs Alice is proving to own. Checking if a key image is unspent is the responsibility of the daemon, not the wallet.

The only drawback is that this forces makers to expose some of the addresses of their Monero wallet. I think this is acceptable. We can make it optional for Alice to transmit a reserve proof. Bob can decide if he wants to accept offers without a reserve proof.

A detailed description of reserve proofs can be found in Zero to Monero on page 82.