Are you sure it was an ACH transaction? I haven't been on ebay for awhile (got addicted. you know how it goes) but the only way I can think of to pay them electronically by any means is through using services such as paypal. You can be debited by Paypal one of two ways (that I know of. I could be wrong) by direct ACH out of your account after being confirmed or by using your debit card.
If the buyer was confirmed and did the transaction as an ACH, 'A' is right, there really isn't much to go on. However, if the transaction was used as a debit card (the quick and easy way IMO) then you could attempt to complete a dispute through Visa (or whomever).
Just a thought. I'm assuming of course...could be all sorts of wrong.
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My opinions are definately my own. I could be wrong. But I don't think so.