Better late than never? Part of why this is late is that I haven't actually ran the Trading Post encounter, so I can't tell you how it plays in practice. What I can do is rant about how it can be used.
Anyway, the idea is that the traders Rannos Davl and Gremag are spying on the village for the Temple. The PC:s find this out by defeating Lareth and finding a letter to him from "RD". This is pretty silly, and I'd suggest using an actual skill challenge to track down the spies after the PC:s have found out that there are traitors in the village. After all, an unsigned letter to Lareth still hints that someone is reporting to him.
The fight itself looks decent. RD and Gremag fill the two stereotypes of thief/assassin - RD is the talkative swashbuckler and Gremag is the dour murderer with poisoned daggers. They have some Human Guards to help them, rounding out the encounter. Without crunching numbers, this looks okay on paper - the Guards keep the PC:s still while RD and Gremag move around and stab them to death. In theory.
I haven't even posted the map (by default, the fight takes place in the villains' trading post), and I'd suggest mixing it up. A sufficiently angered Temple could send a message to the traitors that PC:s are to be killed in their sleep. Grab Daniel Rivera's maps of the Inn of the Welcome Wench and you're good to go for a final showdown in the peaceful heart of Hommlet. Have them bring Zert the Human Berserker along, by the way. He's a spy for the temple too, after all.
Unfortunately, this fight feels very disconnected from the Moathouse (it doesn't take place there, after all). What it does well is connect the Moathouse to the rest of the Temple of Elemental Evil. Whatever the spies do - flee, attack or get investigated - they show the party that the cult they defeated was bigger than just Lareth's outpost. This hopefully incites them to investigate further, and move on to the Temple proper. Or follow whatever nutty plans you have.