Gendar is the Seven-Pillared Hall's resident information broker. He's a dark elf, but, as is the fashion among drow these days, he's abandoned the Lolth-worshipping ways of his people in order to live a solitary existence as an exile.
Characters who took the "Trade Mission" fetch quest prior to coming to Thunderspire will find that Gendar is their quest destination; he'll take their delivery off their hands and introduce himself as a dealer in "old treasures from the Labyrinth". It's unclear whether that's code for "magic items", but it probably is, as there isn't otherwise an item-seller in the Hall.
Gendar's also got a side-quest for the PCs. It's entitled "Treasure Seeker", and it has Gendar tasking players to "recover" an ancient skull scepter from a Duergar in the Horned Hold. It's one of Thunderspire's better side-quests, because (a) it occurs somewhere that the players were going anyway, and (b) "skull scepters" are an item thematically linked to Orcus, which again ties players to what will eventually be the overaching plot of the module series.
I'm actually more than a little frustrated that my players missed Gendar entirely, and never picked up this quest.
The Forgotten Realms conversion for Thunderspire in Dungeon #156 fleshes out Gendar some more and gives him a handful of extra treasure-seeking sidequests. To be honest, Thunderspire didn't need more quest hooks - it's already got way more than the players can meaningfully interact with, given the pacing and XP budget - and these ones are particularly unhelpful, as they send the players to places that aren't detailed in either the module proper or the supplementary Dungeon Magazine material.
Finally, Gendar sells information. In return for big wads of money, or as payment for quests completed, Gendar will hand out information about the Labyrinth and its denizens. Exactly what information he might give out isn't detailed, but you can imagine it might include the back route through the Horned Hold, the location of the Court of Bone, the solution to the Proving Grounds, and maybe even Paldemar's surprising secret master.
Players who decide not to fight at the Trading Post, or who forget to leave some Duergar alive for interrogation, can pay Gendar in order to get set back on track towards the Horned Hold. Sacrificing money in order to not have to fight is the closest Thunderspire gets to a non-combat problem resolution, so anyone who goes this route should take a bit of time to bask in it before moving on into the next round of Duergar-killing.