when it sais that "The perpetrator can attempt to steal a particular type of merchandise by taking a -4 penalty on the proficiency throw." those this include presius merchandise?
IANAA, but I think being able to pick Precious Merchandise could be reasonable. The Common Merchandise table has a 15% for Precious Merchandise so a thief could end up with this result without taking the -4 penalty for picking a specific good. That said, this would significantly increase the value of a successful hijink so I think it's also fair that it carry a higher risk: being caught stealing Precious goods should result in a charge of burglary or robbery, never theft.
You could take this a step further and allow a thief to try to steal a particular type of Precious Merchandise, like Gems. I would apply the penalty again for a total of -8 to the Pick Pockets roll, and getting caught would always result in a charge of robbery.
I pulled this onto a spreadsheet to see what the overall value would be for normal stealing, stealing precious goods, and stealing gems. I assume a sale price of 60% of the average value of goods stolen, and then multiply that by the success percentage. I also ignore demand modifiers or changes in market value over time. I haven't worked out costs for getting caught and fined (or killed!), but I'm already pretty far down the rabbit hole here... It should suffice to say that your chances of being caught don't drop below 20% until level 5 (for precious merchandise) or level 8 (for gems specifically.) I found that normal stealing would be most profitable from level 1-2, trying to steal Precious goods from levels 3-6, and after that point your best bet is stealing gems.
That's a good question - does the -4 include the ability to pick within Precious Merch or not? Given gems are....0.06% of all merchandise, and just taking the average of Precious Merch value is a 5 fold improvement over the average common merch...eh.
If one starts targeting high-value loads, there's always the threat that the Powers That Be may take note - if not immediately direct action, I could see aggrieved parties or their allies paying for their own lawyers and bribes to offset or overwhelm those from the players, chipping away at the syndicate via legal maneuver.
Thus weakened, a rival guild(s) are hired to finish the job, and a good lesson is learned by all. And it's an in-game way to resolve the fact that pushing for advantage in corner cases often gets one cornered. (...gonna have to write that one down, that's good old-timey philosophy)
You can pick "precious merchandise" as a type of good for -4. I hadn't considered picking a specific type of precious merchandise at -8, but it seems like a great rule for high-level cat burglars and jewel thieves. "Only Achmed, Prince of All Thieves, could snatch jewelry so rare...."