Garfield the Deals Warlock

Garfield the Deals Warlock is the sales associate at the Fantasy Costco and handles all of the buying and selling. While early episodes included other employees and mentions of a manager, he now appears to be the only person that works there. In the series recap from The The Adventure Zone Zone, Griffin refers to him as the "proprietor and sole employee" of Fantasy Costco, and in episode 58, he appears to have ownership of the store, so this is likely a retcon.

History
The boys frequently encounter Garfield during their shopping trips and make several deals with him.

Lunar Interlude
Magnus beats him in a test of mercantile wits, allowing him to purchase the mystery bag, the Lens of Straight Creepin' and the Tankard of Potent Drink for 700g. Merle tries to do the same, but, unlike Magnus, doesn't beat him.

Lunar Interlude II
Magnus purchases the Shield of Heroic Memories and the Glutton's fork in exchange for 1800g and his old shield, signed in blood at Garfield's request

Lunar Interlude III
Magnus exchanges his old chest plate and some blood for a rapier.

Lunar Interlude IV
Magnus purchases the Grappling Hook and the Pocket Workshop for 1800g and five hairs from his sideburns. Taako exchanges The Slicer of Tapeer-Wheer Isles for the Flaming Poisoning Raging Sword Of Doom.

Lunar Interlude V
Garfield decides to close down Fantasy Costco for good due to a mixture of the impending apocalypse, and being outsmarted by Taako during the boys' last trip. Needing some of Magnus' DNA, Taako makes one last deal with Garfield, and trades the Flaming Poisoning Raging Sword of Doom for some of Magnus' blood, though he casts Drawmij's Instant Summons on it so he can get it back later. Garfield gives them a key to the back room of Fantasy Costco, where it is revealed, for unspecified reasons, that Garfield was using Magnus' blood and hair to create a clone of Magnus.

Trivia

 * Garfield the Deals Warlock was originally used in My Brother My Brother and Me Episode 255.