Dreaming of buying a knife is a warning sign as per dream meaning and interpretation. The dream means you will not be thinking properly in near future and this might cause hardships. It also means revenge. Dreams of buying a knife means you might cut off a relationship. It also means problems in an association that you are a member of and you might decide to end the association.
Dreaming of buying a knife and you are seen in the dream throughout means you will decide to avenge an insult. It also means getting carried away as part of group and doing something that cannot be reversed.
Dream of buying a knife and it is by someone else means you will face physical attack from jealous person. It also means people might mistake you for someone else.
Dreams of buying a knife and you wake up happy and elated means you will soon be filled with vengeance. It also means suicide and destructive thoughts arising out rejection or other troubles.