How could the seller have done that?

I actually won the auction it said pay now as it always does,so I clicked on pay now and then nothing.

The whole listing has just disappeared.

It might have been deleted by ebay so I'm just waiting to receive some notification from ebay to verify that's what happened.

Is it showing in your purchase history? If not, then I'd say eBay has removed it for being dodgy. Wasn't a $19.99 Makita 18V cordless was it ? 😛

No if you won it, then only ebay can remove it.

But you normally get a message saying don't pay for the item and if you have go to etc. etc.

Could have been dodgey or simply a reported listing violation.