Compagnie Financière Richemont SA, CFRUY 1.36% owner of Cartier and other brands, will pay up to $3.3 billion to buy the shares in e-commerce firm Yoox Net-a-Porter it doesn’t already own. Luxury brands have been slow to embrace e-commerce, largely avoiding partnerships with Amazon.com Inc. AMZN 2.65% and other marketplaces due to concerns about counterfeiting and pricing. But the industry faces the same problems as down-market brick-and-mortar chains, including declining store traffic and competition from online-only competitors. Richemont’s decision shows that neither side has all the answers - online marketplaces need well-known brands for their financial firepower and large customer bases, and the incumbents need help attracting younger, web-savvy shoppers and tackling the complex logistics of e-commerce distribution.