Check out the condition of any bags you buy. If they seem too worn for one use, try the sniff test. An odor of bleach or a "fishy" smell suggest bags that might have bacterial contamination from seafood. As a licensed nurse, I can't bring myself to sell bags used for seafood.
Just in case you're wondering, I buy my bags for several sources, all of whom are coffee roasters or associated with them. Whenever possible, I buy from small custom coffee roasters and do my small part to support small businesses. To the best of my knowledge, all of my bags are food safe. I know my vendors and have established relationships with them.
If you're in doubt about the source of a bag, just ask. While a dealer will not reveal the exact source, they should have some idea of the prior use of a bag. Then decide if a "deal" is worth it to you.