Why does Eliza say she’s no longer fit to sell anything but herself? To what is she referring? (Act 4)

Eliza responds that she was above selling herself when she was a working-class woman; she merely sold flowers instead of her body. Higgins replies that her moral judgment against marriage is unfair. This act also expresses Shaw's deepest condemnation of society, which is fleshed out more fully in Mrs. Warren's profession; that is, he puts in Eliza's words the idea that societal marriage is nothing better than the exchange of sex for money like what one sees among prostitutes. Eliza, if not also Shaw, views the upper-class marriage market as more degraded than her previous profession of selling flowers. From a class perspective, at least, her opinion expresses Shaw's deep socialism, supporting the claim that the working classes can and often do have more dignity than the hypocritical segments of the upper class.