If trade to the surface is limited (save the daring Grelzik!), then you have to think about products that various groups/race may have control over but other races would want too. A lot of the ideas presented above will still work, just will need a bit of thought.
I like the idea of a dispute, but, in theory, this is one where the Pech should have the upper hand if it's strictly a "military" conflict. If they weren't strong enough (either by themselves, or through alliance) to fight off one faction, they wouldn't be in control of the marketplace! BUT even if the troglodytes are not be strong enough to invade the market by itself, they can still put a lot of pressure on the Pech by causing trouble, such as blockading one of the main routes to the market. Or they may have valuable goods that other races desire and their boycott is very bad for business.
I really do like this notion of an underground market, and your motivations for doing it are good. Keep in mind that enterprising PCs can make ridiculous amount of money with such a market. Certain gems and minerals will be easily 50% or less of surface price, and certain surface goods will easily sell for double or triple the price. So a (say) 5 000 gp investment could become 20 000 gp very reasonably.
The reason why this isn't done by regular merchants of course is the dangers involved.... adventure!