Also, to add to this, most pet stores receive fish weekly (at least mine receives a huge batch every Thursday) and the pet store is not always in control. The store itself has fish exchange at such a high rate, and most of the employees do not know what to look for, let alone how to treat it. So when they sell them to you, if a condition is aggravated and surfaces, bring it up with the store manager that there is a problem and make sure the employees are knowledgeable on how to recognize and treat any diseases. If they have the ability to recognize and treat them, then usually they have already taken precautions to prevent such diseases from surfacing with their entire batch of bettas (as well as other fish).
At least I know this happens with my store, I'm hoping this will help. :)