As far as I know, according to the actual orthoepic rules the stress should be on the "o" (it also sounds more pleasant to my ears), but the form with stressed "a" is admissible as well. This means that the form with stressed "a" has not established as a correct one yet, but it might happen in the perspective :)
It is funny to see how different this is treated in the English for Russian speakers course. There they insist that уже is a marker for past perfective and that the word уже itself doesn't need to be translated in the English sentence. There the correct translation would be: Tom had sold his car.