Should I write a letter to home seller?

We recently saw a house, which has been in market since May 2008 this time, according to neighbors it has been on and off market since last 3 years and never been sold and was rented for a while. The house is owned by a doctor for investment purposes. It is in move-in condition and in a really nice neighborhood, there is no specific defect or any problem in the house. We made an offer which was 70,000 below asking price, and he didn't counter offer. We really like the house, but not thinking to spend as much as he's asking because looking at the market because it really is overpriced. In this situation, should I write letter to seller stating why we made low ball offer, and what should I write in the letter, how can I pursue him to sell his house to us? It's not on sell by owner but by real estate agent.