it proves ownership as a valid message can only be signed if the individual signing the message controls the private key of the associated address. ie no one else can sign a valid messag from that address since they do not control its private key; assuming you posted an address say, for a signature campaign to receive payouts to, you could sign a message from that address. as far as potential buyers are concerned who will only care for a signed message, your excuses dont prove anything, and avoiding the question only adds to suspicion of a hacked account. take it how you will, im trying to help you get a sale by encouraging a signed message from you, but youre free to disregard my words completely of course.