BOSTON (Reuters) - Marcato Capital Management LP demanded on Tuesday that Rent-A-Center Inc start the process of selling itself or the activist hedge fund threatened to work to throw out board members up for re-election at next year's annual meeting. The San Francisco-based investment firm, fresh off a victory at Buffalo Wild Wings Inc, is now throwing its weight behind hedge fund Engaged Capital, which for months has been urging Rent-A-Center to sell itself, arguing that an overhaul of the company could be best achieved in the hands of private owners. In a sharply worded letter, dated...