As was mentioned before, players logging in only to find that their stuff is gone, and nothing to be done about it, will likely make them leave (even if it's "only" farm produces). This just doesn't fly.
Defending against thieves and bandits is the core gameplay. It is like playing a kingdom management browser game but logging in to see that you have lost a few battles while offline.
If you successfully plunder someone's house and take something more valuable than X gold pieces, the owners can hire a sherrif/investigator/diviner to find out who did it, so they can hunt you down.
because it's cool to steal everything from some "stupid loser" and walk through town in their premuim armour that they worked for so hard.
Strange as it sounds, gold won't be steal-able. Only farm produce and other trading resources. Also, personal equipment and items won't be steal-able either.
Basically its one big trading game: either you buy goods from one trading post and transport it to another for profit, facing bandits along the way, or you can build farms and etc to produce your own goods, facing thieves.
Although personal equipment/items are 100% safe, I suppose players can get defeated again and again by thieves and bandits and go bankrupt. Even if there are no player controlled thieves/bandit. The AI can beat weak players again and again until he has no resources and no money left.
In that case, there are free resources (harvesting/mining from the wild) or he can join another's trading party and help them fend off bandits. In the process he will be able to grind level and gold, and hopefully beat the AI.