Ernie Garcia III wants to do for used cars what Jeff Bezos did for books, but disrupting the sale of 5,000-pound SUVs is proving far more challenging than peddling paperbacks. Carvana Co., the internet-based used-car dealer Garcia founded in 2012, has become a poster child for pandemic high-flyers whose stocks have plunged back to earth. Over the past nine months, Carvana has seen 92% of its market value vanish, taking with it the majority of a fortune amassed by Garcia and his father, a major shareholder.