Civil Rights Law

"Is I legal . . . " I don't know, were you born here?

Correction
Is it legal for a business to have a policy of selling only to trade union members?

A civil right is an enforceable right or privilege, which if interfered with by another gives rise to an action for injury.Discrimination occurs when the civil rights of an individual are denied or interfered with because of their membership in a particular group or class.

It really depends on the type of business you're talking about. Shops (or other businesses) that are open to the general public may not discriminate against any potential buyer. But there are certain shops that are not, in fact, open to the general public. A discount club, for example, may require its shoppers to become members in order to be admitted to the store. Sam's Club, a Walmart subdivision, is one such enterprise. I can imagine a store being set up in which, in order to be served, one must first show a union membership card and become a member. There are shops for Federal government employees that require identification as a government worker. I can see no violation of law in such an arrangement.