WordNet (r) 3.0 (2006):

buy in
    v 1: amass so as to keep for future use or sale or for a
         particular occasion or use; "let's stock coffee as long as
         prices are low" [syn: stock, buy in, stock up]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "buy in":
   buy, buy back, buy into, buy off, buy on credit, buy up,
   complete a purchase, corner, engross, financier, invest, invest in,
   lay out money, make a buy, make an investment, monopolize, place,
   plow back into, plunge, procure, purchase, put, rebuy, regrate,
   reinvest, repurchase, risk, sink, sink money in, speculate,
   venture