Source: The Great Interpretation of Dreams (Ibn Sirin)
Buying in a dream indicates desire for the thing bought. Buying what is good indicates goodness, and buying what is evil indicates evil. Buying a slave indicates buying worry.
Source: Perfuming Humanity in the Expression of Dreams (Al-Nabulsi)
Buying indicates replacement. Whoever buys something and sells it has replaced one state with another.
Source: Signs in the Science of Expressions (Ibn Shaheen)
Buying is interpreted as goodness. Whoever sees himself buying something will attain goodness.