With so much emphasis in today's market on finding the right stocks to buy, all too often, investors ignore an arguably more important piece of the puzzle: knowing which stocks to sell. Or perhaps it's easier to simply avoid selling when you probably should, whether it means locking in a big gain, cutting your losses, or raising funds to put that money to work in more promising opportunities.

With that in mind, we asked three of our contributors for stocks they believe investors would be wise to sell. Read on to see which businesses they chose.

Steve Symington: It seems an understatement to say Liquidity Services (NASDAQ:LQDT) has had a rough couple of years. Incidentally, I gave the online surplus and salvage auction specialist a thumbs up in my CAPS portfolio in early 2014, after shares plunged more than 30% in just a few days after it lost a large chunk of its U.S. Department of Defense non-rolling stock and rolling stock surplus contracts, and kept the remaining contracts on less attractive terms after bidding reached higher-than-expected levels.

Lqdt Stock Logo

Image source: Liquidity Services.

But things only got worse from there. Almost exactly a year ago, Liquidity Services revealed that Wal-Mart (NYSE:WMT) -- at the time its single largest retail customer -- had terminated its exclusive agreement with the company. Nonetheless, Liquidity Services insisted at the time it would still be able to meet its financial guidance, and subsequently saw shares pop this past February after beating that guidance thanks to the strong performance of its commercial capital assets group.

In its most recent quarter, however, Liquidity Services shares plunged once again after the company revealed mixed results and disappointing guidance for the coming year, thanks both to macroeconomic pressure and uncertainty surrounding some of the terms of a new DoD contract under which it began operating in October.

In the end, while these uncertainties won't last forever, I think the difficult environment Liquidity Services continues to face isn't likely to change in the very near future. So, while I'm happy to keep Liquidity Services on my watchlist to look for signs of a return to sustained, profitable growth, in the meantime, I think our market has plenty of much more attractive opportunities in which investors can put their hard-earned dollars to work.

George Budwell: Sarepta Therapeutics (NASDAQ:SRPT) is a clinical-stage biopharma that has been shooting higher ahead of the FDA's regulatory review for its Duchenne muscular dystrophy treatment eteplirsen. Bulls are excited about this stock because eteplirsen should be able to rake in around $500 million in peak sales. Furthermore, the company shouldn't need to employ an expensive, large-scale sales force to market eteplirsen given that the target market is already fully aware of the drug's existence -- and is eagerly awaiting its approval, according to numerous reports. 

Srpt

Image source: Sarepta Therapeutics.

Sarepta's bull thesis really picks up steam, though, when you throw in the fact that eteplirsen is an orphan drug, meaning the market would probably be willing to pay a significant premium for any revenue derived from its sales due to the host of benefits these drugs confer to their manufacturers. And then you have the more recent development that BioMarin's competing therapy, drisapersen, appears to be DOA going into the FDAs target review date.

As the saying goes, though, if it sounds too good to be true...

Although there is a strong demand for this drug from DMD patients, their caregivers, and many within the medical community, eteplirsen's approval does hinge on a tiny clinical trial that may not be enough to sway regulators this time around. So, as this binary event could easily swing either way, investors may want to lock in some of the stock's eye-popping 150% gains this year prior to the FDA's upcoming decision.

Daniel Miller: If investors can spot changing trends in the consumer world early, and find a way to invest accordingly, it will generally lead to some huge portfolio returns. On the flip side, if you find yourself on the wrong end of long-term trends, you can be in a world of hurt.

When thinking of the latter situation, it reminds me of DISH Network Corp (NASDAQ:DISH), and the graph below shows the devastating contrast:

DISH Chart

DISH data by YCharts.

Consumers inevitably grew irritated with increasing pricing, fees, taxes, and a plethora of cable and dish TV channel packages that, for the lack of a better term, sucked. When the Internet offered tech-savvy consumers the opportunity to kick the traditional TV programming to the curb via Netflix, Hulu, and other options, the new digital trend began, and DISH has been fighting an uphill battle ever since. It's not a trend or a stock I would want to be invested in.

Executives at Dish aren't oblivious, and the company has begun investing for its long-term future by pushing into the wireless business. Unfortunately, many investors believe its push into the wireless business is out of the company's realm and could put the company at risk.

Ultimately, Dish's third quarter showed that its pay-TV subscriber losses accelerated, its customer churn rate -- which is the rate at which subscribers canceled service -- rose from last year, and its total revenue growth slowed to a meager 1.5%. In my opinion, this isn't a trend that's likely to change; it's why I don't own shares of Dish, and if I did, I would strongly consider selling.

Daniel Miller has no position in any stocks mentioned. George Budwell has no position in any stocks mentioned. Steve Symington has no position in any stocks mentioned. The Motley Fool owns shares of and recommends Netflix. The Motley Fool recommends BioMarin Pharmaceutical and Liquidity Services. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.