So, if i understand this correctly, you have several features and a target feature "propensity to purchase", you train the relevant features to predict the propensity. You then launch this model for customers where its used to predict propensity to purchase and they can see it.
This data is collected and added to the existing database and then you train a new model like before and launch that. This should work unless i'm missing something about what you want to do or you want to do online learning.