I gotta tell you all, NOTHING is simple about this. :)
In UO we actually had to take OUT realistic economy sim aspects because they weren't fun. :P Shops being overstocked and not paying for player goods, shops going bankrupt from failure to compete with players, players going bankrupt from failure to compete with shops... and on and on and on. Every tiny change sent massive tremors though the entire game. We're at the point now where it's a lot less realistic, but kinda works, and sort of moves along.
If you expect a player to be able to make money occasionally from selling items they make, you will find that players will expect to be able to sell those items reliably for a reliable price. And telling them that there's a glut of bagpipes in the area doesn't mollify them much.
Some of the things we tracked and did:
There was more, but it boils down to this:
In the real world, you can spend $5 for a block of wood and turn it into a great wooden foozle. And the market in foozles can be so bad you lose your shirt. But in a game, players will say, "labor implies profit! I MUST make money at this!" and they will report that as a bug.
So now we have a less accurate economy, but one that satisfies players. And we learned (again) the lesson to never lose sight of the enjoyment of those who aren't as cutting edge as you are. ;)
In the end, UO began to move to a player-driven economy, whereby NPC shopkeepers are gradually being taken out of the loop. The sole concern is whether newbies will retain sufficient access to the goods they need to get started in the game.