Thanks to Uber’s meteoric rise in valuation, several startups are trying to mimic the idea of building marketplaces with instant gratification. So much so, that there is an aptly titled poem, " Uber for X", devoted to this. Though the jury is still out on Uber or Airbnb, some others like Exec and Homejoy have already failed to be sustainable businesses. Here are a few thoughts on the characteristics of marketplaces, including so-called sharing economy startups, which decides their eventual fate.

Characteristics of an on-demand marketplace

1. Price For a low-ticket item like Uber ride whose average price is 16$, chances of a buyer looking for cheaper alternatives are lower. For a high-ticket item like Airbnb where an average rental is ~100$, the chances a buyer will look for alternatives is higher. That might explain the success of VRBO/HomeAway which undercuts Airbnb with lower fees.
2. Standardization/Commoditization of goods/experiences sold Riders are OK with their Uber drivers as long as the drivers are good enough. And the driver has to provide, more or less, the same service to every customer. That’s not true for a cleaning service like Homejoy, both the cleaner and the cleaning job has a huge amount of variation. This makes the job of providing a consistent service much harder.
3. Interaction frequency The frequency of use should be high, or else buyers or sellers might forget that the marketplace even exists. PiggyBee is Uber for shipping goods, a traveler whose journey matches with the journey of your good to be delivered. Such a service would have a hard time converting and retaining users. This is crucial, a marketplace not only needs users but needs the right balance of suppliers and buyers in time and if interaction frequency is low, it is very challenging to attain that.
4. Low platform leakage The underlying structure should be such that there is no practical advantage for a bond outside of the marketplace. Regular Uber riders can’t just take the phone number of a driver and call him for the next ride. Since the driver might not be in the proximity or might be with other riders. That’s not true for say hiring a home tutor, once you have found a good one, you can take the next purchase offline undercutting the marketplace of its revenue share. That is what probably killed Tutorspree, touted as “Airbnb for tutors”. An even better structure is where taking the dealing outside would be disadvantageous. Consider, for example, buying goods on eBay. It is better to perform the transaction on eBay to get the buyer’s protection. Or, for example, when borrowing money directly outside LendingClub, the borrower will have a little negative consequence if s/he decides not to pay back. Another thing which encourages platform leakage is the lack of urgency. An Uber rider might not want to wait one hour for his/her previous driver to be free, but a homeowner would usually be fine waiting a few hours or sometimes days, for a good cleaner. In almost all cases, though, a transaction can migrate to a similar platform if service being provided is commoditized/standardized. For example, Uber will have a higher chance of losing customers to Lyft than StyleSeat losing customers to LifeBooker, as long the same service providers are not listed on LifeBooker.
5. Trustworthiness While the platform does vet both the supplier and buyer to some extent including providing insurance for the transaction, there might be a further requirement of trust. An Uber rider and driver would have much less requirement of mutual trust that the guest-host pair staying in the same house. The problem is worse for renting physical goods since the lender does not even know if the good returned is the same shape or not. So, a car owner renting a car on RelayRides (now Turo), would require more trust in the borrower than a homeowner would require in an Airbnb host.
6. Fully online vs. online-to-offline interaction Lending club is a purely online experience for the lender and the borrower; Uber is not. A purely online model not only allows for easier scalability but also it makes intermediation easy in case of a dispute. This point, though, is relatively minor compared to others.
7. Goods vs. experience If a dispute arises, it matters whether the transaction involved a good or an experience. It would be relatively easier for the marketplace to be an intermediary in case of a dispute involving a good. When it’s an experience, it is words of buyers against the words of the seller. This point, though, is relatively minor compared to others.

Below is an analysis of some popular marketplaces based on the above characteristics

Note: Some ideas mentioned in this post are based on conversations with friends and are not entirely mine.