Hi antoinelaure,
I sometimes bought items on eBay UK and when I paid for them by PayPal, there was a choice of the currency in which I wanted to pay. So I could have paid in GBP or in EUR (if the seller would have preferred payment in EUR). In your case, the currency shown is either the one of the country where you buy the item or EUR since you registered in an European country.
For certain countries, payment in USD is certainly always possible, so there should be no problem for you to pay.
If you want to pay by other means than by PayPal, you should contact the seller before bidding to make sure that he will accept.
Regards