Task: Question Answering
Input (English): Passage: In 1983 Australia, television repairman Ray Jenkins and his football team celebrate the end of their season by spending the weekend in Thailand. Ray's best friend Gavin, a small-time criminal working for local property owner/crime lord Pat Shepherd, asks Ray to transport heroin on his return flight. Ray refuses, but finds out his step-father is deeply in gambling debt, and his mother will be targeted if he does not pay up. He agrees to transport the heroin. In Thailand, while wandering through the markets, Gavin goes to pick up half a kilogram of heroin to bring back to Pat. Before he leaves, he purchases an extra half kilogram to sell on his own. At the hotel, Gavin hides the heroin in condoms, coercing Ray to swallow them. Upon their arrival at Melbourne Airport, Ray begins to panic and is eventually detained by customs officials. Believing that Ray is a drug trafficker, he is arrested by Australian Federal Police agents Croft and Paris (Hugo Weaving and Ewen Leslie). Ray's lawyer Jasmine Griffiths tells Ray that he can only be held in a hotel room for four days.
During the four days, Ray tries to hold back his bodily functions to prevent himself from being convicted, aided by codeine, which constipates him. Gavin returns to tell Ray's mother Judy and stepfather John that Ray has been arrested. They plan to head to the hotel to visit him, but John has a discussion with Gavin, revealing his participation in the drug scheme to get Pat to get rid of his debts. 
Paris arrives at the hotel room to find Ray being tormented by Croft and a police guard. He kicks them out of the room and comforts Ray, giving him more codeine. 
Question: Who purchases extra heroin to sell on their own?
Output (English): Gavin.