As Scoots said, make sure that “immediate” is selected from the dropdown box when selling to a standing buy order. Some market orders were created under the old market system before the rounding to 4 sig figs thing took effect. So, their prices might be a price that you can no longer actually sell or buy at.
Moreover, this will prevent you from having to pay the broker’s fee for setting up a standing sell order.
And, in case you were wondering, you are effectively creating standing sell orders if you have a time selected from the dropdown box. It just looks like you are selling directly to a standing buy order because as soon as you’d create the order, the game would see a standing buy order of a sufficiently high price, and then automatically sell to that.
Hope that clarifies things.