Perhaps there are data solutions to this that don't rely on gathering user data, but instead analyzing data trends. Purchasing votes, and websites do sell these services, might accumulate very quickly or with reliable time series. If possible, perhaps by analyzing the signal to noise ratio from regular upvotes versus artificial ones, catching them could be done.

This would, of course, require an elaborate set of data analytics combined with an ability to actually view correlations and trends in that data between users to suss out whether engagement is legitimate or not.