(someone) could sell sawdust to a lumber mill sentence
A very persuasive salesperson is able to sell products to those who don't need them.
She has the ability to persuade others, so she could sell sawdust to a lumber mill.
You will need to learn from John who could sell sawdust to a lumber mill.
The origin of this phrase is not clear.
It was often said by a photographer when he would like small children to look at the camera and smile before he took a photo.
When the photographer said “watch the birdie”, all of us looked at the camera and smiled.