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From http://en.wikipedia.org/wiki/Shill

A shill is an associate of a person selling goods or services or a political group, who pretends no association to the seller/group and assumes the air of an enthusiastic customer. The intention of the shill is, using crowd psychology, to encourage others unaware of the set-up to purchase said goods or services or support the political group's ideological claims. Shills are often employed by confidence artists. In the UK the term plant is also used.