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
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.