Have a look at the Anypic tutorial app and see how they use the Activity class. I think you could use this concept to track purchases.
Basically, they use a separate class to track when users like photos. The Activity class tracks what is liked, who is liking and who owns the photo that was liked. A class like this could track any info you need, like purchase price, purchase time etc.
Also, see how they do caching for a more snappy user experience.