I am a complete novice and have no technical skills and little knowledge concerning iphone app development an in-app / itunes store purchases.
But I have been playin
Even if allowed, you'd lose 30% of your revenue to Apple using in-app purchase.
There are point-of-sale apps for iPhone that allow you to swipe credit cards. One of the Twitter founders has a startup called Square that'll be doing this. If you wanted it to be an app that users could install themselves, though, you'd likely be best off doing a custom one and hitting a payment gateway in the backend (Chipotle's app is a good example of this).
The original asker is just asking if this is possible, which it is! If you don't take the term 'in-app purchasing' literally. Just use paypal or some other payment method through the app. Ebay does this and so can you.
As long as you aren't selling something that competes directly with Apple (like Sony's digital book store that got shut down) you should be fine using a payment method other than the built-in functionality called 'in app purchasing'.
You have a good idea on your hands and it should be possible to find a way to take customer's money through the app.
You can't do this according to Apple's terms of use:
You must deliver your digital good or service within your app.
Do not use In App Purchase to sell real-world goods and services.
Though you could probably use In-App purchasing for this, you would have some interesting work to do to make it work smoothly. You would probably be better off developing your own shopping cart application that lets your customers order "online" and write a custom iPhone application for them - skipping Apple for the purchase of coffee and donuts...
-t