In-App Purchases can be implemented into your DBPRO apps; whilst there is no straight forward procedure or official store front development kit or plugin that I can think of, the basis of such a system is placed on transactions which would be processed over internet with a web service such as Steam or Google Play or your very own e-commerce server.
Such an interconnection between your DBPRO app and a store front server is made possible with HTTP or similar functionality or DLLs, there may even be some projects about the subject archived in the forum search.
Not for the faint hearted, DBPRO is not the recommended tool for the job, but it is possible. You may also consider making use of Steam to create a Steam Workshop for your game which can behave like an in-app purchasing system; the challenge is getting onto steam, and implementing the steam API in DBPRO for which there have been solutions revealed in earlier forum threads.