I bought a 7 key from them just the other day after we had to replace the motherboard on one of the PCs in the house due to the old one failing and MS not playing ball. Activated fine and it was indeed a retail key, not a MSDN key or anything. They supposedly bulk buy them and then sell them all off cheap, how they bulk buy in such high quantities to sell them at £20 I don't know, but I don't know what MS offers are.
I specifically bought from s5ean since hes the one that actually runs the page, the other sellers seem to get reasonably good feedback.
Take it with a pinch of salt though, I'm sticking with just the one key from them atm and seeing what happens, but I can confirm it wasn't an MSDN key, or another other offer as far as I can see. Seems to be a proper retail key.