I just don't think it works.. From what I've seen, the ROI on purchased lists is so low. I see much more success having others send me email to their database, than me purchasing theirs.
But I'm with ben, if you do purchase, send out the first email with a cheap service, like mail chimp, so you don't wreck your Marketo reputation. Then if they convert on your form, they'll be auto-entered into Marketo.