Just tell them what you will pay for it and be willing to bolt if they say no.
If it's a guitar that's worth about $1000 and you would not buy it for more than $700, then just tell them that. Don't insult them by saying it's worth less than it is, and don't debate what it's worth.
I would tell them that I think it's worth $1500 because I love it and it's awesome,,,,,,,,,but I will only pay $700 because that's the truth and the way it is.
edit;
Actually I was being sarcastic on the last part.
I would say $700 is my bid because I already have an arsenal of gear but would still like buy it if at a really bargain price.
Oh this is GC? Just run away!