If I understand correctly, you want to allow users can browse freely without logging in. But, a user must signup/login to do bid or buy.
While using Kinvey, if you refer to the documentation - http://devcenter.kinvey.com/android/guides/users#ActiveUser, you will find following salient points:
Kinvey can track usage that way, but doesn't do that.
An active user is defined any user that makes API requests during that month. So, you will be charged for active users (registered users that access the app during that month + one hard-coded user).