Basically, a buy order is when a player places a request for an item or x amount of resources in the market place. The money for the buy order is taken immediately from the player or node when placed, but they can get it back if the buy order is cancelled obviously. Eve Online has well-designed system for this, if you want to look up examples in other games.
1. It promotes trade and crafting
Let's say a guild or node in the mountains need timber and/or lumber. They can simply place a buy order for this in their marketplace, at a premium price. Gatherers and Processors from all over the world will be able to see this, and they can load up their caravans to fill those buy orders. At the same time, perhaps the mountain node has plenty of cheap stone and metal that other nodes pay a premium for, so players can fill up their caravans again for the return trip. Everyone become rich and happy...
2. It promotes Caravan PVP
Obviously with many caravans come the thieving thugs intent on stealing the goods. They can also see those buy orders, and may plan to wtfpwn the n00bz for riches. The guild or node receiving those goods might have to send defenders to protect the caravans, resulting in a grand ole open world PvP fest.
3. It promotes player driven politics and node/castle wars and thus a less stagnant world.
Maybe the above mentioned thieving thugs aren't your random lowlife players. Maybe they are a mercenary guild hired by a competing node, intent on stopping the progress of the mountain node. Maybe said mountain node discovers this, and declare war on both the mercenaries AND the node that hires them, resulting in big battles and much smack ta.. I mean, political discourse.
I think buy orders will add immensely to the game, above and beyond the immediate convenience factor.