SteveH

Know what you are buying (seriously...)
Know what it is worth TO YOU.
Leave emotional attachments out of it (awww how cute)
Be polite
Be honest
Be on time
Be willing to chat a bit.

The last 4 can also get you referrals to other people/machines.

I have had a case where I was very up front with a seller (in person) that they had seriously undervalued a machine, not only did they not try to raise the price but they offered to include the "extras" that they had planned to sell separately, because "you were so nice and honest with us"

EDIT: I forgot to add that I have purchased MANY machine that were too pricey for my budget because of sending a message like the following;

Dear blah blah,

I see you have listed your XYZ sewing machine for the price of $MAJOR and I do wish you good fortune selling it for that price.

However, should you find that you are not able to do so, please consider my offer of $REASONABLE. I have cash in hand and am available at your convenience.

Steve