Ebay send an automatic reminder 2 days after purchase.

An unpaid item case can be opened 4 days after sale date.

I usually send a message at 5 days asking if there is a problem with payment.

If no response, I open an Unpaid Item Case and that can sometimes spur a response or payment.
If not, I close the case as soon as I can, 4 days after it's opened.

Final fees are returned, buyer gets a strike, I add them to my Block Bidders List and re-list the item or offer to an underbidder.

And if you haven't, set blocks so it weeds out non payers.