One major difference is that data is sold tends to get a life on its own. Company A sells to B, B to C, etc. Before you know it, your e-mail address/phone number is in the hands of many companies.

Google is different in this regard. Since they don't sell raw data, it's easier to opt-out (remove data via the Dashboard[1], delete your account, etc.).

[1] http://en.wikipedia.org/wiki/Google_Dashboard

Fair enough, that is indeed a – possibly major – difference, although Google department A sharing data with department B, which gives stuff to department C could be argued to be similar, as the size of these departments easily match those of other companies.