I think 2 or 3 days for payment to be made is reasonable and eBay allow sellers to start a non payment dispute on the 4th day and close the dispute on day 8 info payment received.

Not quite; eBay gives full 4 days before seller can open case, that means you can open case on the 5th day.  Then the buyer gets further 4 full days from the moment they opened NPB case, not a second earlier, before it is possible to close.