29% of Internet users have bought something from their spam

An article on Directmag reports the following:

Twenty nine percent of Internet users have purchased goods from spam emails, according to recent research by online security company Marshal.

The most commonly purchased items include sexual enhancement pills, software, adult material and luxury items such as watches, jewelry and clothing, according to the UK-based firm.

The types of stuff being bought by people sounds reasonable because it corresponds to the amount of spam that our filters see and the number of spam rules we create.  But 29% of people buy stuff from their spam?  Really?  29%?  That seems a little high.

That's a little more than 1 out of 4 people.  Now, I've never bought anything from my spam, and I can name at least 40 other people who haven't, either.  That's not very telling, of course.  My circle of friends is not a random sample.  On the other other hand, Lee Mathews of Download squad reports the following:

Here's the kicker: the survey only involved 600 people. Is it worse that about 180 of those people bought products from spam, or that media outlets are willing to jump all over a statistic that comes from a sampling of less than .0001% of the roughly 360 million people currently using the internet?

The implication here is that the sample size is too small to be representative of the population at large.  But is it true?  If you go to this sample size confidence interval calculator, given a population of 300 million internet users, and assuming that this survey is a random sample, the actual proportion of internet users who have bought spam stuff is 29% ± 4%, or, we can be 95% sure that somewhere between 25%-34% of all internet users have bought something that spammers have advertised for them.

29% is entirely plausible given the sample size and mapping it to the rest of the population, but something to me suggests that the sample was not random.  I simply find it hard to believe that many people buy stuff from their spam folders.