How does sale of fukubukuro work?

Honestly, this is something quite new to me and I really doubt I would get a bag which I will probably not have use for the contents. And since I am a foreigner :) and I can't sell the contents on Yahoo Japan (I was told). But I am interested to know what this is all about.

Questions:
1. how does this •Ÿ‘Ü or fukubukuro work?
eg: shops are closed on 1 Jan 2010; queue starts on 2 Jan 2010? Are the contents known in advanced? I was told Muji's is.

2. Where do you get those bags? Every departmental store or brand's retail branch has its limited stock of the bags?

3. Has anyone had good buys previously? Maybe I can consider one too :) LOL

I think what you're talking about is referred to as a ''Lucky Pack'', somewhat like a ''grab bag''.

The bags are sold at certain stores and contain merchandise worth much more than what the bags cost, but you don't usually know what's inside.

I've heard that sometimes it's hard to get them from certain stores because they're so popular so they sell out fast (it's a limited supply).