When is a penny worth $300,000? When it's one of the first copper coins struck at the U.S. Mint in Philadelphia, in 1793. The annual Florida United Numismatists (FUN) convention is being held in Tampa this week, and three historic coins, including the pricey penny, went up for auction Thursday evening. Mark Borckardt, a senior numismatist with Heritage Auctions in Dallas, said the George Washington-era penny is one of about 500 in existence. The 63rd annual FUN Convention also features 'over 600 dealer booths, a competitive exhibit area, 15 educational programs' and a host of other activities. FUN describes itself...