Given that there was only one bid I would probably give them until the 29th to pay, I would open a dispute immediately if they did not pay on that date. OK so they shouldn't have bid if they couldn't pay but if you either cancel or open a dispute there is no guarantee it will sell on a relist given there was not much interest in it first time round.

If you word your agreement iproperly as if you are doing them the biggest favour ever you may get yourself a repeat customer.

I did that with one buyer a few years ago and she has bought a couple of hundred dollars worth of items from me since, always asks for a bit of time to pay but never misses a payment and uses bank deposit!