[jwoods19]

“In a gold rush, sell shovels”… Well, at some point in the gold rush everyone already has their shovels and pickaxes.

[krupan]

Or people start to realize that the expected gold isn't really there and so stop buying shovels

[gopher_space]

The version I heard growing up was "In a gold rush, sell eggs."

[FergusArgyll]

Selling jeans is the one that actually worked