(Olympia, WA)  --  A congressman from Washington state is looking to do away with marijuana retailers having to sell their product for cash only.  Because marijuana possession is still against federal law, banks and credit unions refuse to back debit- and credit-card transactions at marijuana stores.  Representative Denny Heck says he's looking to pass legislation that would allow banks and credit unions to sanction transactions involving checks and credit- or debit-cards without legal liability.  Heck says he's received bipartisan support for the measure in the past, but congressional leaders have so far declined to get on board.