A) He was selling you medicine instead of fish, right?
B) The only way food would introduce parasites is if you're feeding live food. Freeze dried, flake, etc can not carry live parasites. Overfeeding will increase your pod population, but they are not bad for the tank in and of themselves. (Overfeeding can increase your ammonia levels, which can be bad for the tank.) Parasites will die without a host. They infect the host because they need the host to complete thier life cycle. None of the parasites known to the aquarium industry will live without a host for 8 weeks.
What symptoms did your fish have? I'd suspect they died of ammonia poisoning, not parasites. Even if it was parasites, there's a very high likelyhood that they came from the store with those parasites.