Question to consider though is- that if you had provided service to a prospective customer, wouldn't you want to be paid? Have you had contact with the agent and is he continuing to work for you in your home search?
A few questions that stem from this one - did the realtor you looked at it with after the initial contact provide you an additional information? Did he or she provide insight on the market? Tips on financing? You obviously found the home yourself, but did this realtor help you at all in any other way, other than devoting the time to take you back through a couple of times?
Second, did the agent have a clause in the listing agreement that states that anyone who came through during the course of the listing who eventually becomes a buyer would result in the listing agent being due a commission? Only the seller would know, and even then, the seller should probably rely on an attorney to interpret that clause.
My best advice - use a realtor to buy the home. If you were not comfortable for some reason with the agent who showed it to you twice, choose another agent to represent you and to approach the seller. Not just defending one of my own, but the process is often more complicated than just writing up a purchase agreement. There are inspections and closing processes that you might wish you had some advice on, not to mention the offer and negotiation process. If you end up in a lawsuit because something was not handled properly - that percentage you saved up front might not look so good anymore.