A store is a store. If he sells toothbrushes he has to take them back if the customer doesn’t want them. If he eats a loss on it, then maybe he should reconsider selling them.
Not necessarily. Depending on the country, stores can set their return policy when it comes to non-defective items. I know several major retailers that refuse returns on headphones if they have been opened due to hygiene issues. If the headphones are defective or unopened, then they will accept returns. OP returning a working toothbrush may fall under the same scenario.