What is a reasonable amount of time to wait for payment when an item sells?

4 days. Then you can open a case.

You can file an UPI against a buyer after 4 full days but you can give the buyer longer if you want to. We had two items that the same buyer bought. We had relisted them several times, so were wanting to finish the sale. The buyer had already bought a similar item from us. Day 4, I sent them an invoice and asked if they still wanted the items. They were having trouble with their Pay Pal account. They paid when they got it straightened out.

If we hadn't given them the extra time, we would have had to file a UPI and would have had to relisted and hoped to sell again.

http://pages.ebay.com/help/sell/unpaid-items.html