Not a good idea.
Many sellers on eBay count on buyers making multiple purchases and combining them into one shipment - one payment.
To force immediate individual payment would cause the buyer to pay more shipping charges then needed and the seller to pay extra charges to PayPal for every payment.
It may solve your problem for a few slow paying buyers but would create many more problems for thousands of honest sellers and buyers combining multiple items in each transaction.
If you insist on im,mediate payment, you can list at fixed price with immediate payment required:
http://pages.ebay.ca/help/pay/require-immediate-payment.html