Can a seller on eBay decide not to sell the item after the bidding is over

No, the sale rightfully belongs to the person who won the bidding
But, the owner could keep it anyway they're just not allowed to so they could go to jail.


Another answer
Yes, they can but that's when you go to Paypal and open a dispute to get your money back if you have paid for your item. I agree, it's not good practice and you will receive bad feedback and loose customers.