I am afraid I couldn't agree with this.
It is MUCH more likely to get more often a sale from a link from other site than from a search engine.
Even nicely positioned sites mostly get up to max. 10% of all traffic from SE.
I would like to hear from someone having different experience, even if he's #1 on a SE.

Knowing Google's business professionalism, I know they are very serious in detecting fraudelent clicks and harsh in dealing with violators of their TOS.
I am not saying it is not possible. Your experience with the ROI is a bit disturbing, it could indeed be the case (fraud), but the general reasoning is wrong.
If some webmasters do such things, it would be shortminded - killing the really nice program, for all the parties involved.

I guess it would be possible for Google to introduce (if they don't do it already) tracking of conversion sales rates for any site participating in Adsense.
If the site is notably below industry average, well - goodbye.