On a first tought, shops need some knowledge about taxes to be able to display prices correctly. It may be required to display net or gross prices, this may depend on the customer.
It may also be required to do some tax calculations, this will at least be the case during the checkout process. Taxes may also have to be applied to shipment costs in some way and depend on the customer and the delivery location.
If one also wants to generate invoices from a shop, there might be also further requirements.
Maybe it is possible to find a simple solution to start with and try to be extendable:
Try to not reinvent the wheel: Other shop systems / frameworks will contain solutions to this problem. But also ERP-Systems will contain solutions to this problem.
Maybe it is wise to have a look at projects like Tryton (http://tryton.org).