Really the most important thing you can do with regards to new potential owners is to vet them, regardless of where you advtertise.Ask questions about the care they will provide, do they have a vet, what cage, what food, if they are first time onwers you should be prepared to give them information about how to properly care for the hammies. Um, basically make sure they understand what is expected of the level of care hamsters need, many breeders also have a "return policy" if you like, where if for any reason the new onwer finds them self in circumstances they cant look after the hamster it comes back to you rather than going who knows where.
Also a good way of doing it is to have a list of interested parties before the mating. So essentially you already have homes before the mating has occured, long before they are born. People who want healthy handleable happy hamsters are usually more than happy to wait until babies are ready. then you know that they have homes and you won't be bringing x amount of hamsters into the world with nowhere to go.
A pet shop is definitely no garuntee they will go to good homes at all.