I'm curious as well. Almost seems to be a cross between Pinterest and Woot!. Seems like enthusiasts pick what's popular/hot via voting etc (a la Pinterest) and then the website organizes selling these items at discounted rates due to their bulk purchasing/manufacturing agreements (a la Woot!).
Or I'm way off and just made up this system in my brain. 50/50 chance.
You are right on; I've used them a few times with great success. However, you definitely want to check the prices, especially on ECD items. I've found items that are the same price through Amazon Prime (free shipping). They seem to do really well with fountain pens/ink and gaming things, and hand crafted type stuff (tops, beads, etc.)