I know this is a common way to attract more buyers.. if you sell diet ebooks or some other commercial product. This is a collectors item, I don't think you'll get way more promotion than you have now. Besides, you take the risk that your shop url will appear on obscure spam sites. All for the clicks and sales.

Maybe instead of a referral system you can create a discount for your existing customers. For example a discount on their second order/coin, and a bigger discount on their third order/coin. A lifetime discount. In this way you can activate the real Guldencoin fans to buy more coins at once or placing a second or third order in the near future.

Let the word of mouth do its job..