Black market = illegal. Grey market = “the trade of a commodity through distribution channels . . . unofficial, unauthorized, or unintended.”
In the evening, near the Wudaokou subway station in Beijing (where lots of students live), dozens of street vendors sell paperbacks ($1 each), jewelry, dresses, socks, scarves, electronic accessories, fruit, toys, shoes, cooked food, stuffed animals, and many other things. No doubt it’s illegal. When a police car approaches, they pick up and leave. Once I saw a group of policemen confiscate a woman’s goods.
What’s curious is how far vendors move when police approach. Once I saw the vendors on a corner, all 12 of them, each with a cart, move to the middle of the intersection — the middle of traffic — where they clustered. At the time I thought the traffic somehow protected them. Now I think they wanted to move back fast when the police car went away. Tonight, like last night, there’s a police car at that corner, the northeast corner of the intersection. No vendors there. The vendors who’d usually be there were now at the northwest corner. In other words, if a policeman got out of his car and walked across the street, he’d encounter all the vendors that he’d displaced.