I don't personally request payment on sale as I feel it restricts the market, but I offer other payment options as well and base my decision on allowing a customer to buy, and then pay through whatever option they prefer.
If I was to request instant payment upon sale then I would be restricting my sales to PayPal customers only.
I leave it the required period of 96 hours, send two payment requests within that period, then if no payment or correspondence is received I file an eBay dispute to reclaim the sale fees, and then block the bidder so they can't stuff me around again.
: )