I'm going to say NO, probably not safe, unless the eBay seller is a reputable established seller that sells offline in actual stores.
The reason is that some iPhones described as "new" and "sealed" with "warranties" on eBay can often actually be "refurbished" and "resealed, not the original seal from an Apple factory" and "have a non Apple warranty".
I've read many experiences where users have purchase a "new" apple iPhone from eBay only to find, for example, that the battery doesn't last and that aspects of the product do not work properly. This is because they are refurbished.
If it sounds too good to be true regarding the price, it's more likely to be not a genuine brand new iPhone.