In professional wrestling, the sell is the physical element of making the action appear realistic to the crowd. In other words, it has to do with the acting necessary to sell the storyline. Reactions to moves are often exaggerated, or "oversold", for maximum crowd effect.
The rise of a significant number of "smarks" (fans who are aware of the staging of pro wrestling and many of its other conventions, but still enjoy its entertainment), has changed the "selling" process somewhat. As these audience members are aware of the element of acting, they tend to judge the worth of a match and its opponents largely upon the acting (selling) not being either over or underplayed. Along with booking, the caliber of the sell has much to do with the success of a promotion.
Certain wrestlers have long-established reputations for "no-selling" (generally refusing to sell) or overselling the opponent's moves. Hulk Hogan and the Ultimate Warrior have long been accused of no-selling. Although some consider The Undertaker to be a no-seller, especially in the first half of his WWE career, his longtime gimmick of an undead individual has limited his ability to sell while keeping the character believable. Many fans have pointed to his match with Kurt Angle at No Way Out 2006, his American Badass character, and several other matches throughout his career as proof that he can legitimately sell offense if needed to.
Overselling (or showing too much reaction to a wrestler's moves) was a trait notably done by Curt Hennig. More often than not, however, inexperienced rookie wrestlers trying to sell their opponent's moves are guilty of this. One example is The Rock, who purposely and humorously oversells the Stone Cold Stunner by performing a backflip when one isn't needed. Shawn Michaels' infamous overselling during his match with Hulk Hogan at SummerSlam 2005 was said to be revenge for Hulk Hogan refusing to make the match a 2 out of 3 falls, in order to avoid having to put over Michaels. However, oversells can also be used for a humorous reaction from the crowd (e.g. wrestlers overselling the pimp slaps of Human Tornado).
Mick Foley played on this term after watching a match with Al Snow (whom he has a well established love/hate relationship with) involving Snow no-selling several chairshots. Foley, later in the same show, claimed Snow landed a lucrative deal with La-Z-Boy, which was odd, as Snow never "sold chairs." Foley also used the term in a statement when referring to the ludicrousness of The Rock's transitional move-turned-finisher, The People's Elbow, before a match against him saying, "If you expect me to lay down and sell that abortion, you're about as dumb as I look."
It is believed that on the taped October 16th episode of Raw, special guest star Steve-O of the Jackass movies was not selling the moves that Umaga came at him with, but it is possible that this was done to keep in line with his "stuntman gimmick." Steve-O was seen laughing after several attacks, forcing Umaga to repeatedly come at him in an attempt to stiff him. Other notorious no-sellers include Koji Kanemoto and Tsuyoshi Kikuchi of Japan, who often don't sell for other junior heavyweights in their promotions. Scott Steiner has also been known not to sell for cruiserweights, as he has gone on record to say that he believes smaller wrestlers have no business to be in wrestling.