Can an iPhone app allow a user to purchase a physical product (say a pen or a mouse) from inside an app, without using Apple in app purchase?
Is it allowed by Apple?
You are not permitted to sell physical goods or services using 'In-App Purchasing' since the goods purchased via this method must relate directly to the app using them.
You are free to implement your own payment handling system for selling physical goods though - for instace the PayPal API
If you already have a website for this purpose you could even forward them to that to take the payment - though this may reduce the quality of the user experience greatly.