Shawn posts on his Facebook account a picture of his limited edition Iron Man action figure and says that he is selling it for $2,000. Peter writes on a postcard, "I accept your offer to sell your Iron Man action figure" and places it into the postbox on 4 August. On 5 August, Yang telephones Shawn, who agrees to sell it to Yang for $1,800. Peter is aggrieved. Advise him.