I've found over the years in business that there's a fine line between a low-ball offer that insults the seller and an offer good enough to create interest from the seller, where he or she will negotiate. The buyer can always walk away and return later...of course the buyer risks the item being sold to another buyer. Walking away seems to work with the car dealers Most will chase you out the door...lol. From the sellers side, it's worked best for me to keep a poker face and not offend the buyer when a low-ball offer is made. If the seller creates a negative feeling, the buyer may not return.
Rumors around indicate that cash is king at the shows. I've always found that you're better off not to create the negative vibe by making a ridiculous offer to a seller. If you're a gambler, you can always return to the seller toward the end of the day / show to make the best deal. it all depends how bad you want the item and what is the demand for the item. My dad once said that true value is what a willing buyer will pay a willing seller. Good luck!