[zissou]

eBay's completed listings is definitely one of the best applications of obtaining sales data on the Internet that I'm aware of. Besides that, in some cases there are ways to imperfectly estimate quantities when best seller rankings are available (e.g. at Amazon) -- Chevalier and Goolsbee where the first to suggest this approach back in 2003.[1]

As you mentioned, monitoring half-life is another imperfect approach, but it is of course plagued by false positives (a listing goes away but no sale was made). There was a Google Tech Talk many years ago where some economists took this approach[2], except they were looking at pricing power instead of measuring quantity sold.

[1] http://www.hss.caltech.edu/~mshum/ec106/chevaliergoolsbee.pd...

[2] https://www.youtube.com/watch?v=SfjAezl3-cU#t=27m20s