Hello Gerrit,
I am the copyright holder and the author of Piano Booster. You cannot copy Piano Booster for resale without:
a) having permission of the author (under copyright law).
or
b) abiding by the terms of the GPL licence. Under GPL you can resell PB provided you also make
all the source code freely available including
all the code that you added to PB.
For an explanation on GPL see this link
Open_Source_LicenseGenerally I would prefer you to make your changes to PB under GPL as well but I will contact you off line about any other suggestions you have.
Louis