From [1] "Price discrimination is made illegal under the Sherman Antitrust Act. 15 U.S.C. §2, the Clayton Act, 15 U.S.C. §13, and by the Robinson-Patman Act, 15 U.S.C. §§13-13b, 21a, when engaged in for the purpose of lessening competition, such as tying the lower prices to the purchase of other goods or services."

I'd gather that the same might apply to sale/no sale and not just prices, i.e. you can't not sell something to someone purely for competitive reasons, there has to be 'some reason'.

[1] https://definitions.uslegal.com/p/price-discrimination/

Full disclosure: I'm definitely not a lawyer.