The bookseller told Schorer the drawing, which had been purchased for $30 at a yard sale in 2017, might be a Durer original.
Schorer said he was skeptical, but he visited the owner, who wished to remain anonymous, a few weeks later and was shocked to see the drawing, titled The Virgin and Child, appeared to be the work of Durer, who died in 1528.
"When you're in my world you spend your life looking for unknown things that lead to fascinating research avenues ... and I could see I was at the beginning of something extremely exciting," Schorer told CNN.
Schorer said he spent three years traveling around the world to verify the authenticity of the piece. The art collector said experts agree that technical age analysis places the drawing in the right time period and the piece bears the hallmarks of Durer's work.
Schorer said he believes the drawing could be worth more than $10 million.
"In terms of relative value, I think you have to compare it to other old master drawings," Schorer said.