Don't haggle over the phone, haggle in person with cash in hand. I "believe" another member from out of state was interested in that bike, but he has since walked away. The "I'm getting better offers" line is to advance his selling price. You're in a better position locally and with Winter coming. Bikes sell for less from now until Spring.
I should note up front, this strategy assumes your daughter (if that is the chosen option) is older than 16.
Go in person with 4 $100 bills, 3 $50 bills, 1 $20, 2 $10s, 10 $1s and your daughter/mom/wife/girlfriend (preferably in that order). Put the 4 $100s in one pocket, put 3 of the $50s in a different pocket. Give the remaining $50s worth of bills to your daughter/mom/wife/girlfriend. When you get there and have gone over the bike pointing out all of the problems with it, offer the 4 $100s. As the haggling continues, up the offer by $50 (but don't ever pull more than 1 $50 out at a time. If you can't get the bike for the $550 in cash you had on you and feel that you really want it, turn to your daughter/mom/wife/girlfriend and ask to "borrow" $50. Make sure they eye you with contempt and put up as much of a complaining fight as possible before turning over the last $50. If you still can't get it for $600, walk away but leave your number in case he changes his mind.. Never be afraid to walk out on a negotiation.
I should also note that the amounts mentioned are how I would approach this sale. Feel free to increase or decrease the numbers to fit your budget or perceived value of the bike. If you feel that starting at $400 is "low balling," then start at $450 or $500. If you do choose to start higher, then bring smaller denomination bills for the rest to leave yourself more room to negotiate.
Good luck.