If they are sold as used decks, actually used on the tables, they shouldn't have jokers, right?

Nope, not completely right. Plenty of used decks also use a joker in their game play. A major example is Pai Gow Poker. Pai Gow uses one joker, yet these just get thrown away and do not show up in used decks.

Jokers would have been removed and discarded when the deck was opened and put into play.

Not is the good ole days. Jokers (both of them, plus and inventory/control card and sometimes a calendar or advertising card, typically went back into the decks.

Maybe there are some poker games that use jokers, but the biggest consumer of cards in a casino has traditionally been blackjack, which doesn't call for jokers.

Not a good rule for all "casinos". California card rooms that offer poker and Pai-Gow can be huge consumers of joker needed decks.

Things have been in decline for a while with respect to full deck collecting. Several years ago it annoyed me to buy decks (at the casino) only to discover the cropped corners of the jokers was done with the jokers face down against a face up deck. Made the jokers useless for game play and spoke to a lack of pride. At the same time, red decks were boxed in blue boxes or other colors.

More recently, it is now important to open the decks and count the cards. 51, while close, is NOT a full deck!