I'd also like to note that I'm buying for gold, too.
All of my items can be bought with gold and sold for gold and I even accept sand, cobble and dirt!
Some items (like obsidian) are 0 sum so you can buy it for the same amount as you can sell it!
Most players don't want dirt so it's not something most people will trade for iron or something more valuable but you can actually trade it for iron or diamonds if you have enough in my store. Just trade dirt for gold then gold for iron. Simple.
And if you want some shop space, let us know.