I just started beta testing the 2.0 update to the billing library for the Play Store and a lot of my users are getting this error Google Play In-app Billing API version is less than 3.
Is this just a matter of time before they update or are some devices stuck on older versions or what?
In-app Billing API version goes with the Google Play Store and v3 is actually quite old (at least 3 years old)
PBL will check user's client version and will return this error message if their devices have a very old Play Store installed on their devices. If you're migrating from AIDL to PBL and you didn't check client's billing API version before, you could end up with seeing users reporting this issue.
You shouldn't expect this error message if you're upgrading from PBL 1.x to 2.0 because PBL 1.x is newer than v3 and it will also do this check. You may want to file a bug to the Google team in this case.