Specific laws vary from state to state, but many states require vendors selling foraged mushrooms to have some sort of certification or licensure. This is because there are a number of varieties of poisonous mushrooms that resemble the ones that are edible.
For example, the state of Michigan requires anyone selling foraged mushrooms to have training and be competent in identifying mushroom varieties, or the seller must have employed such an expert to gather the mushrooms for him. The person who identifies each mushroom must be documented with the MDARD Food & Dairy Division.
Some states entirely prohibit the selling of foraged mushrooms. The state's public health department can usually provide information on legality and requirements.