Here's my never fail method to get the lowest price on ANYTHING that says "best offer" or "make offer"- Look them straight in the eyes and ask them "what price would you be comfortable taking for this?" They will tell you their bottom line, and usually you'll get a lower price than you have the cajones to offer.
One example 1966 Ducati 100, 500 o.b.o. I asked they guy what he would be comfortable accepting, and got it for 300 bucks.
Another less succesful, but still good technique is offer a price that is really low, then when they are done stammering in disbelief, they will generally tell you a price conciderably lower than their first asking price.
Good luck, keep us posted.

Jetts