Good platform for retargeting and self running display campaing

Criteo helps us to retarget people who visited our products (contact lenses). Our potential customers are going to visit different websites to compare products and prices, and to remind them of our site and the specific product they saw is the best strategy so that we aren't forgotten. Potential problems are an over showing of our banners, resulting in too much pressing. Also, retargeting a customer who already bought is a waste of money, but could be "part of the game".