Using a USA charger in Europe
If it’s a genuine Apple charger, you should be fine, those are universal. If it’s not (and it’s not unheard of for counterfeit items to be sold on eBay), all bets are off.

See for instance Ken Shiriff’s teardowns of iPhone chargers to see what enormous differences there can be among charger designs and their safety: http://www.righto.com/2012/10/a-dozen-usb-chargers-in-lab-apple-is.html