Anyone showing their hand like that is trying to 'move the market'. Often it's a fake intent and will be removed as soon as shown. If they were intent to buy or sell why show their hand which would give confidence to the novice trader and push the price against them. Don't fall for it! Reminds me of Gordon Brown who announced to the world that he was going to sell half of Britain's Gold holding before doing so, thus surpressing the price and getting a worse deal for Britain. You don't announce your intent before a trade unless you have another agender.
RE: Massive 500 Bitcoin Sell Wall Right Now for ETC at Poloniex