The trouble with "someone selling it online" is that unless it's a bad knock-off (the sort that has a superficial error, as Mike mentions) it will be hard to get the information that will confirm whether it is real or not. The menus there look like the Nougat software on a Samsung, and the phone looks superficially correct. The baseband version always used to be a good way of spotting fakes: they often programmed it to lie about a lot of the software versions, build numbers, even hardware details, to make it look like the real thing, but few tried to fake the baseband information, and as the SoC will not be the one Samsung actually use the baseband firmware will be quite different. Even then I think we've once or twice encountered fakes that covered that base. So without getting your hands on it - which if it really is a fake may be too late by then - it's hard to be 100% sure.