Yes, there is a company called Stripe that offers an API helps do the in app billing without the need to use the Google in app billing.
EDIT :
For the legal aspect of this, it depends on what you are selling , check this from google play policy.
App purchases: Developers charging for apps and downloads from Google Play must do so by using Google Play’s payment system.
In-app purchases:
Developers offering virtual goods or currencies within a game downloaded from Google Play must use Google Play’s in-app billing service as the method of payment.
Developers offering additional content, services or functionality within another category of app downloaded from Google Play must use Google Play’s in-app billing service as the method of payment, except:
where payment is primarily for physical goods or services (e.g., buying movie tickets, or buying a publication where the price also includes a hard copy subscription); or
where payment is for digital content or goods that may be consumed outside of the app itself (e.g., buying songs that can be played on other music players).
The source