Legal analysis: Yes, because they certainly have such a diploma in university, and they also want to earn some extra money. So it's not illegal.

Legal basis: Article 486 of the Civil Law of People's Republic of China (PRC), if the offeree makes a promise beyond the acceptance period, or makes a promise within the acceptance period, it is a new offer if it cannot reach the offeror in time according to the usual circumstances; However, unless the offeror promptly notifies the offeree that the acceptance is valid.