The one initial the order determine whether it is a buy or sell.

I q at 1.00 to buy, you Q to sell at 1.01.

So nothing happens, no transaction. C person come in the market want to sell, but he/she doesn't want to wait, so straight away sell at 1.00, so it is a clasified as "sell".

Can always ask this kind of quesiton in here, been answer before as well. http://forum.lowyat.net/topic/609001/+1040

Cheers.