My first playthrough I fought the gnolls and saved the traders. I never had their chest and let them take it themselves. In turn they gave me the pass phrase to get in. The first time when arriving and trading he only showed me normal stuff and said I needed to talk to Zarys which I did. After that he was more than willing to trade the good stuff.
My next playthrough I let the gnolls kill the traders and I took the chest but didn't open it. Actually I think I tried and failed the lockpick attempt. Then I had to talk my way in to the Zhent hideout (which I'll admit took some savescumming to roll the numbers needed multiple times, otherwise I was always dead on the spot). But once in, I gave the unopened chest to Zarys and she declared us a friend of the Zhents. But the trader still would not offer to trade anything but normal crap. I think that's probably a bug.
