Every time Alexa.com updates, the trading window is closed, thus allowing for all of the prices to update before anyone is allowed to start buying or selling. The trade window generally stays closed for two hours following updates.

The purpose of the trading window is block arbitrage opportunities that may arise due to price discrepancies between alexa.com and urladex.com.
If you perform arbitrage in this game, you may expect some envy from some members of the player community.