Ebay is not a bank or a payment facility, they look at the exchange rate for the day and lock that in.

Paypal constantly monitor and change the exchange rate several times a day as do the banks.

I am sure ebay could employ people to check and change the exchange rate, in hundreds of different currencies, several times a day but do you really want to pay a lot more for your items when they have to up seller's fees to pay for it all?

I use Paypal to transfer money between my UK and Au banks as it costs me less in fees plus I get a better exchange rate than if I do a CHAPS transfer!