What's the word for someone who pretends to buy something from a street-seller, in order to draw other people?

This person is called a shill:

A shill, plant or stooge is a person who helps a person or organization without disclosing that he or she has a close relationship with that person or organization. Shill typically refers to someone who purposely gives onlookers the impression that he or she is an enthusiastic independent customer of a seller (or marketer of ideas) that he or she is secretly working for.

The Oxford English Dictionary notes that this term was first used in 1913, and is chiefly North American. Older terms to refer to these decoys from the OED include:

• stale (now obsolete)
• barnard
• barnacle
• setter
• tumbler