IAP: MS billing simulator for local testing?

Hi,

Regarding local signed APK IAP testing on Android,

what do these lines really do and why do we need them?

 builder.Configure<IMicrosoftConfiguration>().useMockBillingSystem = true;
 builder.Configure<IGooglePlayConfiguration>().SetPublicKey(bouncingAndroidLicenseKey);

where bouncingAndroidLicenseKey is my google play app key. No matter whether i set useMockBillingSystem to TRUE or FALSE, it gives the same result, eg i can successfully buy an item, and when i reenter the app it is then already bought. So how should this "Microsoft IAP simulator for local testing" be used?

Also, how can i clear bought product, so that i can re-buy? My test project is full of bought product IDs that i can not test with again (because those products have been bought once already).