Wiktionary
vb. 1 (context idiomatic English) believe; accept a craze or fad for valid. 2 (context finance English) To buy stocks or shares of (a business).
WordNet
v. buy stocks or shares of a company
Usage examples of "buy into".
It'd be easy to buy into Aberdeen now, easy to begin a calculated retreat from London without hurting anything Edinburgh has all the modern amenities of banking, communications, ports, airfields, that we'd need to operate efficiently.
Already he'd borrowed heavily to redo their house, to buy into the west side subdivision and golf course, and to wheedle a position on the proposed ski valley's board of directors.
Hearst had already proposed that she buy into his new venture, Cosmopolitan Pictures, currently making movies in his own New York studio at Second Avenue and 127th Street.
Louis, she had allowed him to buy into the paper while she kept control.
Way back there in Mexico, I was looking to buy into Seven Seas, Limited.
The first prospectus invited him to buy into time-share condominiums, now under construction.
He had lost his desire, too, to buy into the land development of these mountains.
This was a proposal to buy into the little light bulb shop a down-timer had set up.