At a time of their choosing they can subject themselves to marketing material (yellow pages) or simple word-of-mouth amplified by the Internet. Treating "knowledge of your product/service" as a market commodity is bizarre and has overall negative effects on competition (more money buys more awareness equals more sales).
I used to buy a magazine called "Computer Shopper", which I heard about via word of mouth.
Even now I will go out of my way to watch adverts for things like films I might be interested in.
Need to be careful with word of mouth though, many adverts are spread by word of mouth, especially on the internet where people are paid to say "hey this new $product is great". Those are worse that clearly marked ads.
