Did you sell it through the Geekmarket? I'm guessing this was something you sold through the recent Maths Trade. In a case like this, you have to create a sales entry, and bizarrely the easiest thing is for you to 'buy' it yourself. Unlike ebay and the like, there is no direct connection between you 'buying' something and the purchase process, which is done between individuals through paypal (or any other method).
I think there are some instructions on the Matsh Trade discussion list explaining what to do more clearly and succinctly than I have here if you need further clarification!
No, not a maths trade. I've done those before and that seemed fairly straightforward.
This was for a sale placed on the page for the game.