Consider some exchange, e.g. Mt. Gox, with just two users, Alice and Bob. If Alice places a sale order limited above 90$, and Bob places a buy order limited below 110$, at what rate will the exchange take place?
It depends on who posts the order first.
If Alice posts first a
sell at 90, then Bob posts a
buy at 110, Bob will
buy from Alice at 90, and any excess will be posted as a
buy at 110 (if he wanted to buy more than Alice sold).
If Bob posted first a
buy at 110, then Alice posted a
sell at 90, the opposite would happen: Alice would
sell to Bob at 110, and any excess would be posted as a
sell at 90 (if she wanted to sell more than Bob wanted to buy).