Alternative form of buy when it snows, sell when it goes

(business, stock market, chiefly northern US and Canada) Stock market securities can be expected to increase in value to a greater degree in the cold winter months from November through April than in the warm months from May through October, and profit-minded investors should manage their portfolios accordingly.

(humorous) Being available for sexual relationships with various people is preferable to committing oneself to a single partner.

Alternative form of why buy a book when you can join a library

Do not expect somebody to make a costly commitment if it will give them nothing they do not already have; used especially as a warning against having sexual intercourse before marriage.