No limit, it seems. Ebay has previously allowed a highest bid ever recorded of $10 million. Bidding was halted during the sale while Ebay took steps to ensure that any further bids were from genuine bidders. They failed. A hoaxer made the record bid using a stolen member's ID. See...
I saw a Vintage Car sell last year on EBay for £450,000.
However,as with the (hoax) $10m bid as stated by heathfield,we have no idea if was a genuine bid.
However,the car in question was certainly valued (in Vintage Car websites) at the approx selling price.