You know I was thinking. I'm sure at first the stores will want you to buy any coins they get. You should keep track of the amount and value at the time you bought them. Then over time show them how many Goldcoins they would of had and show them the value if they kept them. For example 1 GLD = 0.00002 when they accepted it and the current value is say 0.00035 or what ever. You can say, you had at onetime 20,0000 Goldcoins over that time I paid you $XXX.XX amount in Dollars. If you kept the coins this is how much more profit you would have made $XXXX.XX. Hope you get what I mean. Show them the profit they could make on top of the retail sale if they hold the coins.
I also think one question will be, How much can I make if I accept these coins? So you'll need to point out the profit that can be made if they hold Goldcoins. Other than that keep the offer to buy them each week or so. And track what you bought, the price and at some point show them what they could have made if they kept them

.... You lost this much in profit over the past 3 months or what ever. You know as well as I do. If you can show them there's profit in it, they will use it.
The goal is to get them to use Goldcoins and tell other retailers about it.
On your flyer have an example:
Candy Bar $0.99 cents = 100 Goldcoins
Hold Goldcoins, watch the market you now you can get $2.00 for the 100 Goldcoins or the 99 cent candy bar just made you $2.00. If you sell the Goldcoins at a later date.
Whats the profit margin on a candy bar maybe 30%?