With my two "new" purchases, the only negotiation in the price it seems was on the "accessories", which was previously mentioned. The Isuzu had the extras already installed, and they wouldn't budge from the sticker price. The Toyota, which took 24 hrs for it to arrive, didn't have the extras, and the salesman knocked off 30,000bht from the list price of the floor room model which had all the goodies. I had the same "extras" added in town for about 14,000 bht. Used vehicles prices are negotiable to a point. However, I strongly recommend if possible, to buy "new". PM me if you want an idea of what to expect with a "used vehicle" purchase.
