[Zope] Help me!

Hello, i'm building a site to sell images (this is the concept, the reality is different...). All the images are in a directory mapped through LocalFS product. For every image i have a record in a SQL table with all the basic informations: author, name of the file, cost. What i haven't understood is how to block users from getting images if they have not enough credit to buy them. I need to control the view method, but i don't know if this idea is the right one or there are better solutions. Any hints? Thanks, Bruno