he shared economy is expected to reach $20 billion by 2020, yet retail’s contribution is evolving. A key challenge is mastering both on-demand service and inventory needs. I suspect the Uber of retail is out there, but it may require a role that the sharing economy has strived to eliminate.

Photo credit: Rent the Runway

Every week, consumers are taking more trips in other peoples’ cars, spending more nights in other peoples’ homes and passing off more of their unassembled Ikea furniture to virtual strangers.

Yet when it comes to shopping for fashion, we are still largely confined to the same model.

The shared economy is expected to generate global sales of $20 billion by 2020, compared with $6.4 billion in 2015, according to Juniper Research. Yet it is unclear just what chunk of those sales will retail will generate. This is because the shared economy concept, while perfect for services like transportation (Uber), hospitality (Airbnb) and chores (TaskRabbit), is not so easily applied to the product-dependent retail segment.

In fact, the issue of inventory is not just a key reason why – it could be the only reason why. Open the Uber app and a driver can pick you up and deliver you to a restaurant within 30 minutes. But how many retailers can deliver three different outfits to your front door, in your size, just hours before the reservation – in the suburbs?

There are shared-merchant ecommerce sites such as Rent the Runway and Le Tote, which deliver selections of clothing that members can borrow for a fraction of the cost of buying them. Yet unless the shopper is in Manhattan or other major metro area, this model lacks the on-demand speed and efficiency of Uber, Airbnb and other direct-to-consumer businesses.

The challenge for retailers may be that we as consumers do not think about the products in our homes and closets as things we can “share” or borrow as needed, rather than own. Shared art, shared tools, shared clothes, shared chairs for a dinner party – this economy exists, but perhaps the consumer mindset needs to change to make it real.

Which forces us to ask: Where is the Uber of retail? I suspect it is out there, still getting its wheels under it.

Sharing Defined: Veruca Salt Economy

In her June report, the Sharing Economy: Opportunities, Impacts And Disruptors, 2016-2020, author Lauren Foye described the sharing economy as cheaper or more efficient than service providers, “cutting out traditional players and providing to customers directly.”

Largely possible through smartphones and other connected devices, the sharing economy’s players enable on-demand commerce. Or, to borrow a line from Veruca Salt in “Willy Wonka and the Chocolate Factory,” it defines an “I want it now!” economy.

“Customers today expect services almost instantly,” Foye is quoted as saying in Retail TouchPoints. “With retail specifically, consumers want products delivered as soon as possible. The key is to meet these consumers’ expectations.”

But how? A number of retailers are trying to prove their models by simply taking advantage of the existing shared economy, not creating ones of their own. They are, essentially, intersecting with Uber.

Cole Haan, Nordstrom, Rent The Runway and Dick’s Sporting Goods have all explored the UberRUSH local delivery service, according to Retail TouchPoints. Walmart and Sam’s Club began testing same-day delivery with UberRUSH, Lyft and Deliv in June, Walmart reported on its blog site.

By delivering for major retailers, UberRUSH is serving as the middleman between retailers and their shoppers.

Sharing The Closet

Not all are taking this route. The crafty retail site Etsy, which shares the talents of artisans, opened a pop-up store at Macy’s Herald Square in New York earlier this year. Etsy had previously collaborated with Nordstrom, West Elm and Whole Foods.

However, such partnerships do not fulfill the idea of a shared retail economy (though at least there’s an Uber to get us there).

Closer to target is the site Rent the Runway, which recently extended its rental offerings from women’s gowns and formal wear to all-occasion wear – even baby showers. For $139 a month, subscribers to its Unlimited service can rent up to three items and keep them as long as they’d like. Items can be sent back and replaced with an equal number of others. Shipping is, of course, free.

The Unlimited model is similar to that of Le Tote, whose members can choose from a broad selection of fashions for $59 a month.

Yet while less expensive and convenient, these retail offerings lack the important “right now” feature consumers increasingly expect. Further, while clothing is shared, these sites do not embody the nuts and bolts of the shared economy model.

Ultimately, the success of Uber and Airbnb is they produce software and logistics for use by their independent agents – creating thousands of individual middlemen. If Le Tote and Rent the Runway invite local entrepreneurs to serve their e-customers, they can factor proximity and speed into the equation as well.

They could, in essence, be more like Style Lend, a site that let’s New Yorkers borrow clothes from others within the city. They just browse through the digital closets of others and receive their requested items in hours.

People outside Manhattan also can borrow through Style Lend, but will not receive same-day shipping. I expect the model can be expanded to apply to different cities, though.

Another promising venture, though the clothing is not shared, is Stitch Fix. This site sends stylized apparel to members based on their responses to detailed questionnaires.

Stitch Fix uses independent buyer partners who source the clothing wholesale from hundreds of brands, according to a review in Business Insider. This enables the merchants to get their products into more closets in a new way, while learning from Stitch Fix’s rich data algorithms.

The “Uber drivers” of Stitch Fix are its more than 1,000 stylists who work directly with the customer members.

Still, the end-users have to wait a while to receive their clothing.

But it is measurable progress. I’d wager that a true shared retail economy is closer to the Style Lend model. Whoever successfully builds on that will have a unique opportunity to tap directly into consumer preferences and insights via their local merchant participants.

With that, they can further tailor their selections for customers and overall assortment.

This article originally appeared on Forbes.com, you can view the original story here.