Go about it properly. Go in, look at it, talk to them, test ride if you can, generally make sure it’s definately right for you and look like you serious about buying it (if you still are after that)
When it comes to the crunch go with the “it’s very nice, but a little over my budget, I had hoped for something about £1,500, but I could possibly stretch to £1,700 for one of these?” That line usually works for me.
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I managed to get a discount on some geabox oil without even trying, asked for a bottle, he gave me one and said “that’s £22 please”. I said “really?” (it was quite a small bottle) and got my card out ready to give him £22, he said “hold on” and typed at his computer, then said “£17.50 is the best it will let me do” so I gave him £17.50 for it.