What happens if two or more people buy postcodes for the same town or city? Take for example Birmingham, you are selling post codes categorised into the following areas:
Birmingham Central
Birmingham North
Birmingham East
Birmingham South
Birmingham West
So if five different people buy one postcode area each, then you are going to have five mini sites/window cleaners to promote on Google for the same key phrase of 'window cleaner birmingham'. And if you promote key phrases like 'window cleaner birmingham central', 'window cleaner birmingham north' etc then it will generate very few (if any) enquiries for the window cleaners concerned because potential customers will not be searching under those terms.