MacFixIt is reporting that Safari users are receiving an error message and they can not purchase items on eBay. When they attempt to make a purchase via Safari they receive an error message "Your web browser may not be compatible with eBay's checkout. Please try again with Mozilla Firefox."
So it looks like users should either download Firefox or keep trying the transaction and it will eventually go through according to MacFixIt.
Here is the full article on from MacFixIt.