Unless it says "Massdrop Edition" like the K7XX I don't think they're limited runs. At its core they basically just organize group buys between people who don't even know each other, even in internet forums, for a discount. What that does for them is that they don't manage a warehouse waiting to offload the stuff in a store - they organize the "drop," get the money, receive the items from the manufacturer, then they ship to the buyers. Once that's done none of it stays in their warehouse.
This is a perfect explanation. I should add that often the discounts are really good. For instance, the HD6XX is a great deal, as was the Elex.