The Stephen King novel featuring Leland Gaunt, the proprietor of a sinister curiosity shop, is Needful Things.
Published in 1991, Needful Things is set in Castle Rock, Maine, a fictional town that appears throughout King's work. Gaunt opens a shop stocked with objects that seem to be exactly what each customer desires, from collectibles to personal charms. His prices are deceptively low: every purchase also requires the buyer to perform a secret prank against another resident.
Those pranks exploit old grudges and private fears, gradually turning neighbors against one another. The novel's central conflict follows Sheriff Alan Pangborn as he investigates the violence spreading through Castle Rock and confronts Gaunt's supernatural manipulation. Gaunt is generally understood as a demonic figure who disguises worthless objects as treasured possessions.
Needful Things is often remembered as the concluding Castle Rock novel, although characters and references connect it to earlier books, especially The Dark Half. It was adapted into a film starring Max von Sydow as Leland Gaunt in 1993.