Question: 做媒 (jyutping) zou6 mui4; (pinyin) zuo4 mei2 literally means "be a matchmaker"

However, there is a Cantonese slang 做媒 /zou6 mui42/ that means:

* be a salesperson's decoy
* a partner of a street peddler or shopkeeper who pretends to be a customer and lures people into buying
* use an apparent stranger to perform a scam
* [literal] to play the middle person (*媒 as in 媒人/ 媒介*)

A scammer's accomplice would be 同黨

My question is: What is the term in Mandarin for "a partner of a street peddler or shopkeeper who pretends to be a customer and lures people into buying"?

Example:

A restaurant's business is bad, so the owner's family and friends pretend to be customers to make the place look good to passersby. These people are all "媒" /mui42/. What they are doing is "做媒" /zou6 mui42/

*My guess is 充場客, but it sounds like Cantonese too*

Answer: 
Maybe, 托儿(read as 托r, not the one in 托儿所） is the one you are looking for? It's someone who helps lure customers into buying.

This use of 托 is relevant to the sense of 烘托 and 衬托(serve as a foil). E.g. 我衬托你 means I serve as a foil to you. So, 托儿 serves as a foil to 商家 in this case to promote products by pretending to be a real customer and etc.
I would say "冒牌"- fake brand, is closer to 做媒 in Cantonese, except while it means the "matchmaker".

他是冒牌的 and 他是冒牌貨 both mean "he is a fake...." for the purpose of enticing sales, or carrying out a scam by faking his identity.

Unless the meaning has been expanded, in Mandrin, 托儿 means putting a child under someone/someplace (other than oneself/one's place) for care, such as 托儿所 and 托儿院.