'Shopaholic' is a common slang term for someone who loves to shop. The technical term for a compulsive shopper is oniomanic, but calling someone a shopaholic generally does not imply any sort of mental disorder, merely that they enjoy buying things. It may at times be applied to someone who simply enjoys to window shop, although this is less common.
The term is a portmanteau of 'shop' (the verb) and 'alcoholic' (the noun). It is arguable that this is a very politically incorrect construction, as alcoholism is generally a more serious condition than consumerism, but this sort of construction gained popularity in the 1960s with chocoholic (1961) and workaholic (1968), among a slew of others. By the time shopaholic was coined in 1984 this sort of thing was already widely accepted without comment.