Apple will not approve such an app, unless users are purchasing something which cannot be purchased via an in-app purchase.
For example, a pizza store is allowed to collect credit card details to pay for pizza's. But a digital music purchase must use the in-app purchase system.
From apple's documentation:
There are four supported kinds of products that you may sell using
In-App Purchase:
- Content includes digital books, magazines, photos, artwork, game
levels, game characters, and other digital content that can be
delivered within your application.
- Functionality products unlock or
expand features you’ve already delivered in your application. For
example, you could ship a game with multiple smaller games that could
be purchased by the user.
- Services allow your application to charge
users for one-time services, such as voice transcription. Each time
the service is used is a separate purchase.
- Subscriptions provide
access to content or services on an extended basis. For example, your
application might offer monthly access to financial information or to
an online game portal.
Anything in that list must use the in-app purchase system, and anything else must not use it.
By the way, these things have changed many times, and could change at any time in future.