eBay contracts are enforceable in Australia 

The Sydney Morning Herald reports that a NSW Supreme Court judge has upheld an eBay auction contract ...

A Mr Smythe "accepted" the reserve price (the "buy it now" price?) of $150,000 for a WWII Wirraway aeroplane seconds before the auction closed.  Mr Thomas, however, refused to deliver it up as he had reached agreement with a third person on a sale price of $250,000.

SMH report here.