I'd lowball them and then negotiate up from there (unless they take your lowball offer, in which case good for you!). If they don't give you what you're looking for, be prepared to walk away. If you walk away and they don't stop you, likely they gave you their true best price and you can either go back later or get it someplace else if a better deal can be found. Odds are they'll stop you before you hit the door and say "well now wait a minute, let me call the boss and let's see what we can do".