Eggshells in our diet
Consumers are buying crushed eggshells for up to $17/lb, even though they can get them for free at home. It's one of many products that turns easily-discarded waste into something valuable, and that derives its pricing power not from selling something that can't be acquired elsewhere but from assuring the buyer that they're getting exactly what they need.
Crushed eggshells are popular as a source of dietary calcium. They can be added to pet food, or to smoothies for humans. In part, this is the same trend that's occurred in the beauty business, where products like caffeine eye creams call out a familiar ingredient like tea or coffee, rather than the chemical itself. Brands know that customers are more familiar with foods than with the chemicals they contain and that something you’d already feel comfortable putting in your mouth is something you’ll feel safe using to wash your skin.
The zero-waste movement also provides some motivation for eggshells; people are used to throwing them away, and knowing they can get a health supplement and reduce food waste is appealing.
Good results for companies
On the business end, the food industry has low and volatile margins so if there’s a way to turn a cost into revenue, or even offset it a little, it can have a huge bottom-line impact. Food companies have been trying to get more use out of their products for a long time: the Wilson sporting goods brand was formed by the Wilson meatpacking company at a time when footballs were literally pigskins. Since US egg production totals to 300 million eggs each day, there's ample supply, and pricing on eBay and Etsy shows that there's significant demand for crushed shells.
Online sellers' biggest challenge is competing with the DIY market, which they do by selling egg fragments in standardized sizes so that pet owners don't risk accidentally letting their pet swallow a large sharp shell. Sellers will often warn against DIY’ing eggs for exactly this reason: when the product can be made at home for free, a big component of the sale process is convincing people to pay anyway.