No-one can tell you whether they are legit or not until they have bought from Jsfordays.com (which I haven't) , but there are 2 warning signs that I have noticed:
1) everyone is asking themselves whether the site is a scam. That doesn't sound too good. The internet is full of discount retailers, ebay, etsy and many other ways to spend your money.
Why is Jsfordays.com being singled out so frequently if they are legit?
Could be a slip up, but it's not very reassuring!
As for whether you're going to get scammed or not: Personally I'd just get my Nikes somewhere that i feel confident about.