Gus Williams -- a two-time All-Star for the Seattle SuperSonics -- is being sued by a man who claims the ex-NBA champ didn't pay him for a house ... and trashed the place to boot.
The man behind the suit is Todd Mustin, who says back in 2017 ... he put his 5-bedroom, 5,000 square-foot Sherman Oaks, Calif. pad up for sale -- when things went awry.
Mustin claims he sold the house to a woman named Megan Zucaro ... but when she only coughed up $1 million of the roughly $1.