Though there were many merchants in Eberron, none could compare to the sheer scope and economic might that is the Dragonmarked Houses. The Artificers and Artisans of House Cannith produced goods of a quality and quantity unmatched by any other, and the Khorvarian (half elven) members of House Lyrander and the humans of House Orien shipped these goods all over Khorvaire and beyond. Their dominance of the market was unmatched. Until recently.
No one is exactly sure when it started, but a growing number of independent merchants began popping up all over Wroat - the capital city! They offered goods of a quality rivaling that of the House Artisans, but at lower prices. No one really minded too much, not even the King - except for the Dragonmarked Houses that is! Slowly at first, but then faster and faster they began losing customers due to the simple law of Supply and Demand. As if it weren't bad enough the Houses had a hard enough time moving their goods, it seemed their goods were being targeted for theft and destruction more than usual as well. This went unnoticed for longer - after all, it could have simply been a crime wave, or otherwise unrelated but by this time the Houses had lost the bulk of their business in Wroat.
And now, in Sharn, the biggest city in all of Breland and perhaps the world of Eberron the same thing is happening again. This time though the houses were more prepared. They even begun to find, and reobtain their stolen goods from some of the suspicious merchants. It was harder than you might think - after all, not everyone not affiliated with a dragonmark house is automatically against them, and harassing innocent men and women would not do well for their reputation. Before they moved, they had to be sure. Of course, having the stolen goods doesn't prove that those same people did the stealing - they were just as likely victims as well, which is exactly what the guilty parties claimed. Of course the Houses didn't believe them, but without some sort of objective proof the law was of limited help. As such, the Houses were allowed to conduct their investigations, and given considerable leeway in doing so but without solid proof - and lots of it, that there was some sort of conspiracy to usurp them economically and perhaps in other ways as well they could not act as a group against these people.
And this is where the PCs come in. You might be a member of House Cannith, Lyrander, or Orien but if you aren't you have been hired by a joint organization of these three houses to get to the bottom of this mess. While the Houses as a whole are greatly limited in terms of what they can do directly, individuals are allowed to do as they will as individuals and even the indirect backing of such giants is considerable - after all, there is much to be said for transportation, and magic items, particularly when you are an adventurer. While it's not all on the corporate dime, rest assured they pay their elite forces quite well. Of course with high pay comes high expectations, and they fully expect to not be disappointed in their investments. They also don't suspect that will be a problem. After all, they paid for the best.