Why not just ask them, "Are you, guys, official Microsoft reseller?"
In case they will answer you, and answer is "Yes, we are", then - they are legit.
But I would not keep my hopes high.
Go to Newegg (legitimate reseller). Go to Amazon (legitimate reseller). Heck, go to Microsoft's own web site and check the prices for their software. Any place that is selling the same software at a significantly discounted price has obtained those licenses through illegitimate means. There is no way for them to make a profit unless the licenses they sell are bogus.