someone interested in buying and asked the lowest price.
i told the price.
that person agreed and sent me a message.
i didn’t reply the message.
that person came to me with the money..

is there a contract? how can i reject legally?

No contract. It is as if you have placed an advertisement with a price; you are not obligated to sell to the first person (or anyone else) who shows up with money for the stated price. It is merely an offer to enter negotiations.

When the person agreed to your price, that was their indication of an interest in pursuing a contract, based upon your solicitation of interest, not an acceptance of an offer.