Seems legit. They buy license at bulk and sell it.
There are many aspect to this.
First of all, there is a license which is a deal between a manufacturer of the software and the entity who bought those keys (assuming that key dealer bought them lawfully). This license often directly forbids further sale of such keys. So according to manufacturer they can be "pirated".
Secondly, there is a law applicable to your jurisdiction. Eg in EU resale of used software is always legal and cannot be taken away by the license. So according to the law they may be legal.
Another issue here is what proof of purchase you'll get from the key dealer? Your local jurisdiction may not recognize this transaction at all without a proper receipt or invoice.
This only skims the surface, because later you might want to exercise rights from the license (eg right to tech support or updates), however the license might be already violated. So you might end up in situation in which the software is legal to poses but not fully-functional (or perhaps not functional at all).
Finally, there is the question of good faith. Were you acting in good faith buying those keys? If you have the knowledge that keys are stolen, then you're committing a different crime here, sometimes even "worse" than pirating.
There is nothing legit about this site - if you go to this link on it they claim they "sell only new OEM software". This is a breach of the Microsoft EULAs.
Further, look at the section titled "How are you able to sell your software at such low prices" in the FAQ and you will see they have no agreement with the manufacturers - which means they can't be negotiating a different EULA.
That the products don't come with a COA should also raise alarm bells - although this is not, by itself sufficient to say they are not leget.
Further, there is no meaningful contact details provided, and the way the site is set up is on the cheap (Godaddy as the register for a start), with hidden domain registration. Also, merchants of a large enough scale to get prices down as low as these guys claim would not be using (only) Paypal to accept payment. FOLLOW THE MONEY !
You can try to convince yourself that it is legal. If you have moral objections against using Microsoft's software without paying if it is illegal, or your wife / husband / parents have such objections, then even if it is legal, you know that Microsoft, who created the software, isn't getting any money, whether this site is legal or illegal.
If you want to rationalise your own piracy, don't. I don't care what you do, but if you are going to use software without paying, at least be honest to yourself.
If you want to know whether you would lose in court in the (unlikely) case that you are taken to court for copyright infringement: All justifications that you can come up won't help you in court.