What I do is use the classic negotiating technique. Knock them down on the price (in person works best), then when they won't move any further, make it clear you are a serious buyer and have the funds ready to go. Then write down your best offer on a business card, hand it to them. Thank them for their time and leave.
Last time I did that within ten minutes I got a phone call accepting my best price!
Last time I did that within ten minutes I got a phone call accepting my best price!