As investors, we tend to focus most of our time on figuring out which stocks to buy and when to buy them. Often, we neglect an equally important task: deciding when to sell stocks.

Recently, I unloaded my full position in one stock, and I'm considering doing the same with another. Read below and I'll tell you what these two companies are, and at the end I'll offer you access to a can't-miss special free report.

Where's the moat?
In early 2012, I decided it was time to part ways with Netflix (Nasdaq: NFLX). Overall, the investment wasn't too bad, as I purchased shares when they were in the $50s. But my decision to sell had nothing to do with price, and everything to do with a lack of sustainable competitive advantages.

Most people may not realize this, but back when the company was focused on DVDs, the location of fulfillment centers -- within walking distance to the nearest post office -- allowed for amazing efficiencies in getting movies out to customers.

It was very difficult for competitors to mimic this strategy, as it would have required a massive spending spree. Amazon.com (Nasdaq: AMZN) essentially has the same advantage in the retailing business, as the location of its fulfillment centers throughout the U.S. guarantees its dominance in e-tailing for decades to come. The fulfillment centers represent a sustainable competitive advantage.

But though CEO Reed Hastings pulled off the transition to streaming with breathtaking skill, the paradigm shift eliminated Netflix's sustainable competitive advantage. It's not like anyone can just start up a streaming outfit, although Amazon has used its Amazon Prime service as a doorway to offer streaming. In the end, though, it's the studios -- not Netflix -- that own most of the content. The studios have a financial incentive to either develop their own streaming business or make sure Netflix pays a pretty penny for content.

That alone, though, didn't cause me to sell. I'm actually willing to overlook the lack of a tangible competitive advantage when there's smart management in place that's executing its plans flawlessly.

Believe it or not, I actually believe that Apple doesn't have any sustainable competitive advantages, but I'm still willing to invest in the company because of the culture and execution that it has demonstrated. I guess you could say that the culture and management are the competitive advantage -- even though it's not necessarily sustainable.

But my once sky-high confidence in Hastings has severely eroded. First there was the price hike. Then there was Qwikster. And when the company announced plans to issue more shares this winter, I'd had enough. Though the company could surprise me, I think there are better places for my money now.

Shape up... or else!
Ever since its IPO last year, I've been a huge fan of Zipcar (Nasdaq: ZIP). I believe the company stands to benefit from a new sharing model by utilizing one of the least rational purchases we Americans make: expensive cars that are only in use for an hour every day.

There's no doubt that Zipcar has a competitive advantage in that it's the first mover within the industry, but competitors are piling in quickly -- like Hertz (NYSE: HTZ) and its On Demand service.

Competitors alone, however, aren't enough to scare me off. Zipcar has an enormous head start on the others in the field. Instead, it's the fact that competitors allow users to drive cars point-to-point -- instead of round-trip -- and Zipcar doesn't.

As a Zipster myself, I can attest to the inconvenience of the round-trip requirement. When my wife and I want to go to a friend's house, catch a Cubs game, or head out to dinner, we'd love to use Zipcar. But because we'd be paying for the car to just sit there for hours, we often use public transportation. If we could use a service's point-to-point option, we'd be far more likely to patronize its business.

Though Hertz offers this, the inconvenience of its pick-up and drop-off locations limit the appeal. Instead, it's Daimler's car2go program that really has me worried.  As Foolish analyst Lyons George put it after interviewing car2go's CEO Nick Cole: "Members simply locate a car on their phone, hop in, take it where they need to, and drop it off -- anywhere in town -- when they're done." Cole later said that cars tend to naturally regulate their locations -- spending nights in residential areas, and days in business-centric settings.

Zipcar can say what they want about customer loyalty, but if car2go's service were available on a large scale throughout the U.S. -- it has operations in San Diego, Austin, D.C., and Portland -- I have no doubt that it would be far more popular than Zipcar.

A keen eye toward the future
For my money to remain invested in Zipcar, the company will either have to prove to me that it's working on a point-to-point option soon, or convince me that car2go's reliance on Smart Cars -- the only brand it uses -- won't attract enough users.

My eye is keenly on the sell button, and the company has a shrinking window to address this threat.

If I do decide to sell my shares of Zipcar, I'll be keeping my eyes open this earnings season for other places to invest it. To help me, I'll be getting a copy of The Motley Fool's special free report: "5 Stocks Investors Need to Watch This Earnings Season." If you'd like a copy, you can get it for a limited time, absolutely free!