Lady Anne's seduction is quite unbelievable, and is perhaps one of the flaws in the play. One moment she is cursing Richard and loathes him for killing her father and husband, and the next she seems to accept his proposal.
Richard does not have appearance and physical beauty on his side, but the weapon he does have, is evidently much more powerful. Richard uses language and words to victimise himself in his exchange with Lady Anne. He twists each of her accusations against him quite subtly, so that they mean something else, which he then uses to further his own case.
Lady Anne is perhaps not seduced by Richard himself, but by his evil allure. In the play as a whole, Shakespeare has hit the nail on the head in terms of human psychology. Humans are attracted to the forbidden, attracted to evil. That is what makes Richard such a compelling and almost attractive character to the audience. It is likely that Lady Anne also succumbs to this same allure of evil. Indeed, she almost seems to be under a temporary spell, for when we next see her, she laments her marriage to the Duchess of York and Queen Margaret. That again brings me back to the point that her seduction is highly unbelievable because she changes her mind within moments! However, it is possible that although she hates Richard, her inferior position in society as a woman prevents her from doing anything but submit to him, and only when she is in the safe company of fellow women does she retaliate against him.
It is among the more remarkable scenes in Shakespeare's "Richard III": The monstrous title character seduces Lady Anne beside the bleeding corpse of her father, whom he has only recently butchered along with her husband. She spits in her suitor's face and calls him "lump of foul deformity," but in due course lets him slip a ring on her suddenly pliant finger. He gloats over his seemingly impossible triumph. Richard seduces Lady Anne to gain power through her familial connections. He then rumors her sickness and impending death in order to kill her.