Couple of things that I keep in mind when negotiation are 1) I try to figure out how long they've had it I n stock, 2)I'll check out GC site and run a search using the word "used" along with the name (sometime used items will be left out if you don't use the word "used" in the search) to see if I can find it cheaper as leverage for negotiating 3) sometimes you can figure out the price they bought it for if they use the old pawn shop labeling system "black horse" (each letter is a number b=0, l=1, a=2, etc. if you see a bunch of random letters on the used tag, chances are it's code for how much they bought it for.
The more information you have they better equipped you'll be with negotiating. If you're a good player, I'd play in front of the store owner. Music store owners typically cut bigger deals if they know you're a monster player. They want you to come back, it's good for business.
Most used music stores buy it at 50-70% of their selling price. How much they're willing to discount will be based on how long they've had it, current market prices, market supply/availability, and how much they like you as a person/player.
So onto the actual negotiation; figure out how much you want to pay, I usually will ask to play the guitar, they'll leave you alone and when the owner comes back let him know you're "really interested in the guitar but was hoping to pay around xxx amount, is there any flexibility?" And see what the come back with.
Right now though, the used market is flooded with gear. I think all the kids returning to school are selling their gear off. Prices on recent acquired used items at GC seems to significantly less than the same items they bought/priced during the summer. So this is a great time to buy used gear.
Good luck
