Nicondel (Nick) Riordan, a half elf with Arcane Trickster and Bladesinger wizard levels. He had a legitimate career in appraising magic items for auction, but used is as an opportunity to fence stolen goods and find deserving targets to rob and eventually had to flee to Sigil, which he also eventually had to flee after various heroic exploits there. He has a powerful magic shortsword he improvidently stole that works as a key to portals, but which he is cursed to be unable to ever have out of his reach by powers which only the very highest magic could possibly sever, and it's wanted by all manner of people for all manner of schemes.
I volunteer him because I myself have used him as an npc in multiple campaigns. Basically I roll him out whenever the party first reaches a major city after reaching a level where a brisk trade in low level, and perhaps a few high level, magic items seems appropriate. He, now fairly high level and retired from adventuring, has returned to his roots in auctioning off magic items and other rarities from across the multiverse. While his business is primarily geared towards auctioning off luxury goods to the very rich at swanky events, he is happy to welcome adventurers into his warehouse to buy and sell magic items. His prices and inventory are based on what he can sell to wealthy collectors at auction, so he might have a very low price for a practical but undesirable item but a steep price for something pretty or that has a compelling story. Of course sometimes he just had a successful auction and his stock of anything interesting is very low, when the appropriate to the campaign. Because he is inseperable from a macguffin sword which many powerful entities are hunting him across the multiverse for, his business may abruptly relocate to some distant reality on short notice. He typically has a low-class theives-cant infused manner of speaking, but code-switches abruptly into a well pracitced and refined luxury goods salesman persona when dealing with the monied.
He also, because the original character had expertise in Arcana and was a homebrew half-elf varient that got advantage on rolls identifying magical items, has such massive bonuses to identifying or figuring out the lore of a magic item that when I'm running him as an NPC I just consider it an autosuccess unless it's a very high DC. We leaned on him as an exposition character in that original campaign, and as an NPC he's had a long career stealing, fencing, appraising, and trading magic goods on multiple planes, so he is basically a walking DMG magic items section, and (despite or perhaps because of his one embarassing mistake with the shortsword) is handy at sniffing out curses as well. On the rare occasions he doesn't know something he is embarassed but always has the spells Identify and Legend Lore at the ready. He also may be knowledgable about local elites he has dealt with, and particularly what ones may be deserving of burglary.
