Right now on poloniex. Eth-USDT has a sell wall of 286.28 and a buy wall of 284.44. It's not even a two dollar difference. Why do they do this? It's this way on about 10 coins that i've looked at.
i think every people have their own reason to buy and sell so they place their order buy and sell and locked in that position. i think its more than 10 coins because i see in poloniex every coins have buy and sell walls but there is a good spread that we can use to follow the price and i think we can take profit from that price.