The Test Market is a series of interconnected pocket dimensions created by Jim Darkmagic for the purpose of screening Acquisitions Incorporated interns. Candidates would enter this dimension through the front door of the Dran and Courtier Inn (D&C) if they carried with them a magical golden acorn, which would recreate for them alternate versions of the Inn, which they then needed to escape. The final 'level' was a place of chaos, where thoughts became reality.
Other appearances (Spoilers for Seasons 2 and 3)
Pocket dimensions exist on the Astral Plane, which is also the home of the Sanguilith. Although it had never been explained on the show, the Sanguilith appears to be able to tap into the Test Market. Early in Season 1, K'thriss and two others were transported out of one version of the Test Market and found themselves before the Sanguilith. The entity destroyed the other two and claimed that K'thriss had made a deal with it that he could not remember, before K'thriss found himself returned to the other members of the C Team.
Later in Season 2, after the C Team captured Oak Truestrike and escorted him back to the D&C, he managed to escape into the Test Market. The C Team went after him and found themselves in a version of the D&C where patrons were consuming drinks made of the Inn itself. When they exited it into the Astral Plane, they climbed into a boat and eventually found themselves inside the Sanguilith's lair, where they discovered Oak entrapped within one of its crystal pillars.
In Season 3, when the C Team and Evelyn Marthain confronted the Sanguilith from aboard the halfling fleet of starships, it caused them to have a shared hallucination of being back in the D&C, with Evelyn having taken Donaar's place. Similar to the Test Market, Onyx was present in this version, and she was able to tell them what was real and what was the Sanguilith's illusion.
 C Team S1E1, Franchise Opportunities
 C Team S1E3, In Good Standing
 C Team S2E2, Plans Within Plans Part 2
 C Team S3E23, The Velvet Cloak, Part 9