they need to have record of instructions, either in writing via a letter or fax from you, or if it's a telephone order, they do record their orders on recording media, sorry is an admission of guilt, the broker has to repurchase your shares at his/her cost.
Happened to me once, via a fax order, the broker put a wrong sell order on, costing me $$$$. he had to buy back. needless to say, this took 6 weeks becasue they denied knowledge of the fax.
