Mr A sent an email to Mr B offering to sell him some goods for...

Mr A has acted in breach of contract as Mr B has accepted his offer

Mr A was free to sell the goods to Mr C as communications by email are presumed to show that the parties did not intend to create legal relations

Mr C has intervened in the contract of Mr A and Mr B and can be forced to return the goods for the benefit of Mr B

Mr A was free to sell the goods to Mr C as Mr B's email amounted to a counter offer which rejected Mr A's offer