I believe that this was done away some time ago to encourage buyers to go through eBay checkout when paying, which makes more sense than going through PP.
When buyers go through PP and link to the eBay sale, that has created mistakes at times. Also, the eBay system was never able to reconcile a PP payment that was initiated the way you wanted to do it. If you tried, it would have switched back to the selling currency.
Of course, when you are trying to invoice in a currency other than the listing currency, this is a drawback. We run into this as well and just use "goods" and reconcile manually.
Bernie