sell banner position (ads)

Hi,

Well, it depends what you want.
You can sell anything. Just create a product, enter a price and the name of the product and you can sell that.
The question is the delivery of the product.
If it's an ad in a place of the website, it will require that you receive some information about the ad from the customer and then place it manually where you want it. To ask that extra information, you can create custom fields of the table "item" via the menu Display>Views (only available with the Business edition).
Note if you want a system that will automatically set up that extra information in the ad location selected by the customer, there is no mechanism for it in HikaShop. That would require the development of a custom plugin catered to the specifics of your project.