I see this taking place in forums a lot... someone has multiple IDs and uses one ID to post a WTS thread and another identity to play buyer... and the second ID sings praises for the first IDs service, ethics, communications, hairstyle, etc. That has actually led people to court in a few cases (see "Fraud in the inducement"), and pretty much as a rule leads to bans from forums. Nobody wants to see you grade your own papers, and it doesn't make you popular to pretend nobody's smart enough to notice.

It is also used in other venues... such as a recent thread here where a person is asking for somone to do both questions and answers on Yahoo Answers and toss out favorable recommendations of their product.

Just as forum mods have a LOT more practice at catching puppet posters and better tools to detect them than the average user thinks... Google is fully aware of the practice and will bust you for it. The word "undetectable" makes them laugh... it's not that the user will get busted IF detected... it is WHEN. They are Google, and as the Bible clearly states...
Quote Originally Posted by The Good Book of Google
"Verily I say unto you... screweth thou not with a corporation that doth have more assets than God on high, lest they smite thee mightily and banish thy site from the face of the web; for yea, mighty is their wrath, and deep their memory for all that hath pissed them off. "
But hey, don't take my word for it... here's the scoop on it from the talented Mr Cutts: