buy in

Verb

 * 1) To accept an idea as valid; to join in on a concept.
 * 2) To invest as part of a group; to put one's personal stake in an investment.
 * 3) To buy back for the owner at an auction.
 * 4)  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.