You don't need any funds in your PayPal account in order to make a purchase. In this case,PayPal will send the payment to the seller in the form of an echeck,drawn on your bank account. These take as long to clear as a paper check and the seller won't ship until the payment clears.
If you still want to cancel,you can't do it. All you can do is contact the seller and explain the situation. Ask if he will offer you a mutual cancelation request. Tell him you will accept if he sends it. The seller does not have to make the requesting he doesn't want to. He can choose to open an unpaid item dispute if he chooses. If he does,and you still don't pay,you will receive an unpaid item strike.
Most sellers have automatic blocks against buyers with 2 or more strikes in the last 12 months.
It's much better to not buy on eBay if you can't afford to pay what you buy on eBay.
