buy in

To stock (something) by buying inventory from a third party.

To accept an idea as valid; to join in on a concept.

To invest as part of a group; to put one's personal stake in an investment.

To buy back for the owner at an auction.

For the buyer of securities, whose seller fails to deliver the securities contracted for, to buy the securities from a third party and demand the difference in price from the original seller.

Support; agreement; blessing (in a secular sense).

The amount that a player buys in for

A tournament where a player must purchase all of his or her chips before the tournament starts.

A protest in which participants buy an item and return it soon after in order to slow down the store's operations.