I can guarantee that if you do this your app will be suspended from Play Store. It might not happen immediately, but at some point they will notice & you'll get pinged for violating the payments policy. If you're lucky, you'll get a strike against your developer account. If you're unlucky, your developer agreement will be terminated.
The only time you can get away with not using the Google Play billing system is if you're selling physical goods or services, or taking payment for a credit card or utility bill. If you have a question regarding Google Play policies, the best thing is to read the actual policy. If you ask the question on a forum, you'll get a bunch of different answers, most of which will be wrong.
Google Play Payments Policy
- Colin.