the listing needs to have a photo of the item with your screename, tomshardware and the date written on it. multiple photos are best.
you also need to list how you want to accept payment, shipping info (or local pickup only and if so what area).
i would also suggest listing how old the system is, the current condition and other important information.
to add an image... use the img tags
[ i m g ] this is where your url goes to the photo [ / i m g ]
without the spaces. or if you use the full edit (not quickedit) you can use the icon to simplify this.
edit: you cant host it here... but you can use
www.tinypic.com which is entirely free