[patio11]

A certain percentage of retargetting does hit customers who have already bought the software, either because they don't exclude previous customers or because the initial lead, the decisionmaker, and the person with the credit card might be three different people.

This is largely considered to be an acceptable inefficiency in light of retargetting's (generally pretty impressive) results.

A related minor inefficiency: I'm doomed to a lifetime of being trailed across the Internet by the ghosts of consulting clients past, because logic (which I, erm, may have implemented) suggests that my behavior means I'm teetering right on the cusp of buying their stuff. I mean, look how many times I went to their website, signed up for the trial, and did virtually everything I could possibly do short of plunking down $X,000 for the product!