The 1966 strip was bought for $50 at a garage sale 39 years ago by Laureen Picciani, a pastry chef from Fort Bragg, Calif.
"This piece of cartoon history could easily sell for $30,000 or more," ComicConnect.com Chief Executive Officer Stephen Fishler said in a statement Tuesday.
Since Schulz's death 10 years ago, original "Peanuts" artworks have sold well, the Web site noted. A similar Sunday "Peanuts" panel that sold for $1,200 in 1993 recently changed hands for $65,000.
"I knew I had stumbled across something special but until contacting ComicConnect, I had no idea how valuable it really was. I was really shocked," Picciani said. "I kind of identified with Charlie Brown because I often feel like I don't know what's going on either."
Currently, the highest bid is $17,700. The auction is to close Monday.