You're example is obvious because Newegg can't tell the difference between market research because you're about to buy versus research because you're about to sell.

But it's always amused me (in a not very amusing way) that immediately after I purchase something I'm inundated with advertisements from the same vendor trying to sell it to me again. Big data is supposed to be about finding statistical relationships between events, right? In what world would the probability of someone buying something be highest when they're holding a brand new one in their hands?

What it really makes me think is the web advertising folks are pulling a con on their clients. They're not actually doing the critical analysis they promise but simply sorting by linear distance in n-dimensional space and hoping no one notices how useless the measurement is.