Negative feedback so they pay higher auction fees (if this is ebay you speak of, not sure why its forbidden last I checked its only forbidden to promote items your selling)
and leave a comment stating their shady practices, and report them to ebay themselves. Unless the shipping delay is mentioned in the auction itself.
IF they have the item in stock there is no reason to not ship it immediately.
I've had sellers try and pull similar BS I reported them to ebay to straighten it out