80% rule
When barting for old cars, I go by the 80% rule. You dont want to go too low, unless they are one of those people that always think what they have is valuable. They can't sell a person "sentiment", because their sentiment has no value at all to you. His price is already fairly reasonable, so you dont want to go too much lower to **** him off. I tend to offer 80% of their asking, which is usually taken. Less than that will depend on the situation, and their "attitude" or depretness to get rid of it. If they put a price of $600, like in your case, they already decided they wanted $500 for it, and added the extra bill to barter with. So, I am pretty confident you can already get it for the 5 bills, so try just a bit lower to feel him out.
Just more food for though.
the sentiment thing reminds me of when people always advertise something as "custom". Well, if something was cutomized to their specs, it doesn't mean that "custom" has any value to anyone else. it is custom to them, they can't sell that, because it isn't custom to the buyer.. lol.. Just sayin. When a seller means, "it means allot to me..". Well at the risk of sounding like a jerk, sorry about that, you can't sell me what it "means' to you, sell me the item, not your attachment to it. And if it meant "that" much to you, you wouldn't be selling it.. Also just saying, lol
And..
Personaly, as a buyer, I don't care when someone says what they "have in it..". All I care about is what I am willing to give you for it. What you have in it is of no consiquince to me. IF you have reciepts proving a $$ amount you have it it may, or may not influence what i would be willing to give for it, but if they dont have that proof, it's only sales chatter and I dont even really hear it..
Again.. Just sayin.. <grin>