If it is, this is a GREAT deal!
I know in britian they use something called CMC powder. But considering that shop is selling suplies to make fireworks, I wouldnt trust it at all to be used in food consumption.
It looks like it's the same chemical, but I know with other things like glycerine there's some that's food-grade and some that isn't. When you look up "cmc powder usp" or "cmc powder food-grade" you get some that say they're intended for food use. I think that's probably important here.
I looked it up and i found one for you on ebay that is food safe, not quite as good of a deal, but still pretty good....
Notice that the first item is listed as "sodium" CMC. According to Wikipedia:
Following the initial reaction the resultant mixture produces approximately 60% CMC plus 40% salts (sodium chloride and sodium glycolate). This product is the so-called Technical CMC which is used in detergents. A further purification process is used to remove these salts to produce pure CMC which is used for food, pharmaceutical and dentifrice (toothpaste) applications.
So, the first listing needs further purification to be food grade. The second auction mentioned by goddessa12 appears to be food grade. Most cake stores sell 55 grams (a little less than 2 oz) for about $4.