Reader's question:  

Do some research ahead of time to know exactly what your item should look like. If a vendor only accepts cash, this is also a red flag.

Could you explain "red flag"

My comments:

Red flags are often used as a warning sign. In the old train station for instance, if there was something wrong preventing a train to enter or leave the station, sometimes a red flag was raised. If everything was alright, a green flag was waved.

In a public beach, when the waves are high or when, say, sharks are detected, red flags are sometimes put up to warn swimmers not to take to the water.

In the example from the top, the fact the vendor accepts only cash is described as "also a red flag" because the vendor might be operating what is called a fly-by-night operation – taking your cash in the morning and fleeing the city by nightfall.

This might not be the case of course but at least you're warned to take extra care. It pays. Better safe than sorry, as they say.

Related stories:

Call balls and strikes

Stare in the face

Cyber Monday

With a grain of salt

Drive home

Out of nowhere

Shove it under the carpet

the sky is the limit

hit it off

get my rib in my heart

With bells on

No harm, no foul

Pick up the slack

length and breadth of

get up to speed on something

Borrowed time

On a shoestring

Shake a leg

Sing to a new tune

Go to Zhang Xin's column

本文仅代表作者本人观点，与本网立场无关。欢迎大家讨论学术问题，尊重他人，禁止人身攻击和发布一切违反国家现行法律法规的内容。

Zhang Xin has been with China Daily since 1988, when he graduated from Beijing Foreign Studies University. Write him at: zhangxin@chinadaily.com.cn, or raise a question for potential use in a future column.