The fast-spreading blaze at the U-shaped wood and metal structure in Cabell County was reported around 11 a.m. Wednesday. Milton Assistant Fire Chief Steve Vititoe says ammunition from one of the booths went off.
By midafternoon, the building had collapsed.
No injuries were reported. The flea market is only open on weekends.
Every fire department in Cabell County was called to the scene, along with some firefighters from neighboring counties. One fire truck received significant damage while fighting the fire, said county Emergency Medical Services Director Gordon Merry.
The venue opened in 1989 and grew to become one of the region's largest flea markets with more than 300 vendors who sold antiques, collectibles, crafts and jewelry.
Milton Mayor Betty Sargent estimates the market draws about 10,000 visitors each weekend.
''The fire has really devastated this community,'' she said.
Nina Roberts of Barboursville ran a candle shop at the market for three years. Because it's difficult to secure insurance for a flea market-based business, she said she'll be taking a loss on her $25,000 to $30,000 worth of inventory.
''I am watching everything just go up,'' she said.