What, if anything, can i do about this item?

[thrifty]

I've sold a few things on ebay lately, but the last item to sell on sunday (cotbed duvet cover/matching curtains and bumper) sold for a pittance. I am very disappointed, even more so when i checked the postage now its parceled up I have underestimated the postage by £3 (its only made £5.21) so by the time I've paid paypal and ebay, driven to the post office etc, I'll be left with diddly squat.
Anyway, the buyer hasn't paid yet (i know its only 3 days, but most people pay quickly). how long do i have to give them to pay? (part of me wants them not to pay so i can sell it elsewhere)
I know its unreasonable to want cancel the transaction just cos it didn't make what I'd hoped, and its my own stupid fault for not checking the postage properly, but is there anything i can do, or do i just write it off to experience?

well, you will have to accept his payment, if it ever comes. give him 5 days then send a reminder that you want it within next 2 days after. then he will have had a week to pay, and inform him that youwill re-list if payment not received by x date.

he has seven days to pay. If he does pay you'll have to put it down to experience.

Give them a few chances, and do so through the eBay messaging service. If no response after the first contact, offer a "compromise" of scratching the transaction. Then offer that "compromise" again, with a deadline.

Check eBay seller's centre for the dispute resolution procedure on how to finally cancel the transaction, so this doesn't remain open. You don't want this person to have the opportunity to say anything about you just for the hell of it, on a non-transaction!