Will buy and sell offers from one trader be matched with each other? If one user made a buy and a sell order at the same time, would they be matched with each other or remain pending?
On "real" exchanges Trading Against Yourself or Wash Trades are usually forbidden. Either by a real time check when you enter the second order or after the fact with fines by the oversight agency.

As Bitcoin Exchanges are not regulated, it's their choice to either block you from doing it or not (earning fees for the exchange).

The spread is pretty wide, so it will probably cost you quite a bit of money to do this on a meaningful scale.