Imagine: You are the seller from France,Finland or other EU euro-zone country, You sell items via eBay.com or eBay.co.uk
So all invoices are automatically issued in US$ or GBP while later on You might be in need to use currency exchange into EURO for
withdrawing and to lose on that some 3-5% on PayPal's rate... Same way Your customers on eBay.com or eBay.co.uk very often can be EU
citizens with PayPal account in EURO... Paying Your invoices in "wrong" currency they also spend to exchange same 3-5% on PayPal's rate... Why not to invoice them in EUROs? They save 3-5% on nothing and You also save 3-5% ...
Same "picture" with US sellers selling via eBay.de or eBay.co.uk and having US customers there.... or UK sellers via eBay.com or eBay.de
And as well YOU as a buyer can offer Your seller to accept the currency other than on eBay site listed...
All that, works in case when buyer trusts the seller, because I'm not sure that in this case PayPal buyers protection policy would work.
In questional cases see www.xe.com/ucc  (copy and paste) International currency converter