Express Inc., best known for its presence in American malls, has been testing a new strategy: selling other brands’ merchandise online.Visitors to the retailer’s website can shop for items from dozens of other brands in addition to Express apparel. That approach is an increasingly popular one as retailers like Express, Urban Outlifters Inc., and J. Crew Group Inc. look to benefit from listing products that are sold and shipped by other sellers. The goal is to increase the chances shoppers can find what they are looking for on the Company’s site, boosting web traffic and revenue without veering too far off-brand. Amazon.com Inc has been leveraging the power of third-party sellers for years, and large-box retailers including Walmart Inc. and Target Corp. have created their own marketplaces, too, albeit on a lesser scale. Smaller, more specialized chains had been uncertain about trying the marketplace model, in part for fear of losing their brand identity in a virtual marketplace.The retail world’s testing of Amazon’s marketplace approach has generated revenue for certain hosts and exposure for certain sellers but has also created a whole new calculus for businesses. Host retailers need to decide whether they dilute their own brands by featuring others. Likewise, sellers need to figure out whether being on these bazaars helps boost their brand-not to mention their profit, once they pay for the lead. And for shoppers, marketplaces can be confusing if they aren’t sure who they are ordering from or what to do if issues arise.