buy straw hats in winter

Especially of stocks, to buy when both demand and cost are low so that one may then sell when demand and price are high. A phrase attributed to Russell Sage, a 19th-century American investor and financier. Primarily heard in US, South Africa. I made a fortune buying shares in the startup company before smartphone technology became ubiquitous—I bought straw hats in winter, and now they're worth a fortune!