Back in the vaudeville/burlesk days (no, I'm not quite that old) they used to sell treats during intermission, like boxes of popcorn, at prices that were (for the time) outrageous, promising that there were prizes hidden in some of the boxes. Of course, when the selling started, nobody would buy, but soon after the selling started, someone that bought a box of popcorn would stand up and display a shiny new watch that he "found" in the box. Immediately, sales of the popcorn would go through the roof.
In reality, this "winner" was employed by the theater to give people the false impression that there was something to be won. In fact, he just bought the popcorn, took the watch off his wrist, stood up, and claimed he had just won. In fact, the none of the boxes contained anything but popcorn. The person that performed this little sales deception was known in the business as a "shill".
When I was a senior in high-school, I actually got to see this performed first-hand at the burlesk show at the state fair. As badly as it was done, it worked like a charm.
So, a "shill" can best be described as someone who is employed by the seller, or has something to gain from the sale, who provides (potentially) misleading or false information under the guise of being a legitimate customer, in order to increase sales.
