It’s better to use the “best price” method here, rather than making an offer, just as you would at a flea market. You never know how long a seller has had a piece in stock, or whether business was off that week and they really need a sale to pay the rent. The price they quote may be far below what you expected since they'd rather make the sale at a marginal profit than let you walk out empty handed.
You're always welcome to make a reasonable counter offer if they don't go quite low enough. These folks are prepared to play the game with you. Sometimes the dealer will accept your counter, other times not. If they say no, it never hurts to leave your name and number and tell them to call you if they change their mind.
Take a look at more strategies for bargaining in different buying venues: