From Longman Dictionary of Contemporary Englishbuy something ↔ in phrasal verbBUYto buy something in large quantities Companies are buying in supplies of paper, in case the price goes up. → buy→ See Verb table
Examples from the Corpus
buy in• Battle Mountain first approached Hemlo about buying the company in 1994.• When you buy stock in a company, you are volunteering to ride the roller coaster of risk and rewards.• They could not buy tickets in advance, so they queue like docile cattle.• Although the under - 18s can not trade shares themselves, adults can buy stakes in collective investment funds on their behalf.• It was as if he had bought shares in my future and were determined to make the trade come right.• It is a common mistake to buy large trees in the belief that they will give quicker cover.• Many pastoral and voluntary caring relationships are now bought and sold in the market place.• The companies bought the shares in the third quarter of 1995.From Longman Business Dictionarybuy something → in phrasal verb [transitive]HUMAN RESOURCES if an organization buys in parts or services, it gets them from another company, rather than produce them itselfUnder the new system, schools have to buy in the services of advisers and librarians. → buy→ See Verb tablebuy-inˈbuy-in noun [countable] FINANCE when a company buys its own shares from existing shareholdersSYNBUYBACKA buy-in has the advantage of removing small shareholders and frees the parent company to run the unit. → management buy-in 