You're not allowed to sell items for solely RSGP in here. Other than that, I doubt this is the best place to be selling your RuneScape rares for RSGP. This is a blackmarket forum, where people sell RuneScape items for Real-Life money, or a mixture of RSGP and Real-Life money. If it's RSGP you're after, I bet you'll have more response in the RuneScape forums.
Other than that, very impressive to have a cracker. I'd never sell it if I had one, unless it'd be for a bunch of $$$. If you were after such a sale, I recommend to be dealing with highly trusted traders only. Watch out for imposters.