Stated buyer must pay within 24hrs, sent invoive, 4d later no payment. What to do?

Honestly, whatever ebay says in their rules...whether it be you have 7 days to pay, you have to wait 7 days to file NPB...etc...it is common courtesy. If a seller states in their auction that they have a specific time frame that they would like payment, follow directions if you bid, period!
 
Ebay can't control what you put in your auction as far as terms of payment. Unfortunately they do control the NPB process wich actually can cost you 16 days if your buyer never responds. If you resell the item before you go through ebays NPB process you leave yourself open for a non performing seller. If the buyer decides on the ninth day (2nd day of dispute)to pay through paypal, and the item is no longer available, they can file against you because that is ebay policy.

If you are sure that the buyer doesn't want the item, and you want to relist now then contact them and ask them to mutually withdraw from the transaction. You don't have to wait to relist, but he won't get a strike either.