The best thing about the stock market is that you can make money in either direction. Historically, stock indexes have tended to trend up over the long term. But when you look at individual stocks, you'll find plenty that lose money over the long haul. According to hedge fund institution Blackstar Funds, even with dividends included, between 1983 and 2006, 64% of stocks underperformed the Russell 3000, a broad-scope market index.

A large influx of short-sellers shouldn't be a condemning factor to any company, but it could be a red flag from traders that something may not be as cut-and-dried as it appears. Let's look at three companies that have seen a rapid increase in the number of shares sold short and see whether traders are blowing smoke or if their worry has some merit.

Company

Short Increase, April 30 to May 15

Short Shares As a % of Float

SolarWinds (NYSE: SWI)

48.6%

3.1%

UnitedHealth Group (UNH 0.23%)

45%

1.9%

Endeavour Silver (EXK -0.73%)

41.8%

1.8%

Source: The Wall Street Journal.

The winds of change are blowing
It's been a longtime target of my hand-crafted TMFULOI index, which seeks out the market's most fundamentally overvalued companies, but cloud-based information technology and infrastructure management software firm SolarWinds appears as if it may finally succumb to short-sellers.

As my Fool's Rex Moore has opined previously, SolarWinds' razors-and-blades model is primed to set the company up for success by locking in recurring revenue and creating the impetus for upgrades as newer technologies are developed. I, however, have felt that the valuation of SolarWinds had gotten way ahead of itself, selecting it as a strong sell candidate yet again in February with its revenue growth set to fall by about 10 percentage points, from 36% to 26% year over year.

Those fears were realized in early May, when SolarWinds reported 22% revenue growth in the first quarter but highlighted that it's having trouble selling new licenses. Part of this weakness could be from businesses that are being particularly cautious about expanding, given the uncertainties associated with the upcoming implementation of the Patient Protection and Affordable Care Act. It could also stem from precipitously high unemployment rates and the threat that the Federal Reserve's paring back of its bond-buying program could stall growth. Either way you cut it, this high-growth company is losing its rapid growth rate, and that trailing P/E of 37 is now exposed for investors to see.

While I certainly give credence to Rex's view over the long run, I think further downside may be warranted.

No Band-Aids needed, but thanks, anyway!
All of the reasons for UnitedHealth Group shareholders to be concerned about growth in their stock were certainly evident in the company's first-quarter earnings report. In spite of an 11% increase in revenue to a whopping $30.3 billion and strong international enrollment, weaker patient volumes in hospitals and a huge 13% increase in medical costs actually caused earnings to fall. When added to the uncertainty surrounding the PPACA, which caps the medical loss ratio at 80%, you have what equates to a can of gasoline and short-sellers holding the match.

In actuality, though, things aren't nearly as bad as they might appear despite these higher medical costs. I ultimately suspect that Obamacare isn't going to be able to control premium costs from rising dramatically, as not all insurers are going to participate in each state's exchanges. With individual insurance falling into the hands of just a few companies in each state, the prospect of higher premiums is a real possibility.

Another factor many are forgetting is that these health-benefit providers will simply jack up premiums well ahead of the implementation of this bill. Take Aetna (AET), for example, which boosted premiums by as much as 21% in some states over the past year. Aetna has proved time and again that pricing power lies with the HMOs regardless of what policymakers of the PPACA would like to believe. This would bode well for UnitedHealth, which can simply raise its premiums to cover medical cost inflation.

With a growing international presence and a well-diversified business, I wouldn't suggest betting against UnitedHealth Group.

Being proactive has its perks
It's definitely not difficult to see why short-sellers are piling into Endeavour Silver, with the spot price of silver having fallen by roughly 55% from its peak. With labor costs rising and spot prices falling, it wouldn't be out of the question to anticipate that Endeavour's bottom-line profit estimates will fall. However, Endeavour is implementing cost-saving plans and focusing its efforts on the recommissioning of its newly built El Cubo plant to drive growth, and it should, therefore, be able to stave off much of the pessimism surrounding the sector.

Announced earlier this month, Endeavour will be reducing its 2013 capital investments budget by 20%, initiating layoffs, and reducing its administrative expenses, with the entire management team furloughing 10% of its compensation. Investors often forget that miners can cut costs and run lean while still turning a profit.

Then there's the company's El Cubo mine, which it purchased in 2012 from AuRico Gold (NYSE: AUQ) with a combination of its own shares and cash for up to $250 million ($50 million is contingent on mine productivity over a three-year period). As fellow Fool Christopher Barker has commented previously, AuRico did a disastrous job of managing the El Cubo mine, which leaves Endeavour with a tremendous opportunity to reap the rewards of AuRico's failures.

With Endeavour having improved total production for eight straight years, I think today's share price is ludicrously inexpensive and would strongly suggest not shorting at these levels.

Foolish roundup
This week's theme is all about the success of being proactive. Both Endeavour and UnitedHealth Group are doing a good job of trimming costs where they can while positioning their companies for success down the road. The same can't be said just yet for SolarWinds, which seems confused over why license growth appears to have slowed.

What's your take on these three stocks? Do short-sellers have these stocks pegged, or are they blowing smoke? Share your thoughts in the comments section below.