Strong, forceful persuasion. Not only a sales tactic but any situation where one person is trying to force another to do something against their will. (Compare to Hard nut to crack. Contrast with Soft sell.)

“I didn’t buy the auto because the salesman was using a hard sell on me and it turned me off.”

“If he won’t  join our union at work, than we are going to have to do a hard sell and give him no option.”