Is is legal for seller to accept another offer after agreeing to take the first one?

[Yinan Q.]

Negotiated with the seller back and forth a few time and finally seller agreed to take my counter offer yesterday. However, seller agent told my agent that they've accepted another offer today before we send in our paperwork.

My question is: is it legal/ethical for the seller and their agent to do so? My agent has all the emails and txt msg about the negotiation and them agreeing to take our offer.

Thank you for your comments/suggestions!