There are a lot worse ways to get investment ideas than tracking the buys and sells of the world's most successful investors. After all, billionaire investors like Warren Buffett, George Soros, and David Einhorn have been there and done that through thick and thin.

The good news is that professionals like these three legends have to file a document known as a 13F with the SEC every quarter that shows the investing world what they've been up to in the previous quarter. In my last article, I discussed what billionaires have been buying. This time around, let's consider what stocks they're giving up on.

Warren Buffett
Buffett is legendary for a reason. He's become the second richest person in the United States by embracing a long-haul investment strategy that is laser-focused on owning great companies for decades instead of days.

As a result of his long-term focus, keeping an eye on the stocks he sells in his company, Berkshire-Hathaway (NYSE:BRK-B), can be very telling.

In the most recent quarter, Buffett dumped a handful of stocks, including small positions in Express Scripts and ConocoPhilips. Buffett owned less than $40 million worth of stock in each of those companies, which made them relatively small positions given that Berkshire has $108 billion in assets under management.

His selling of Express Scripts suggests that he's less interested in owning pharmacy benefit managers and their razor-thin margins for as long as he holds other stocks in his portfolio. This theory seems to be backed up by the fact that Buffett profitably traded in and out of Express Scripts PBM competitor CVS Health in 2011. If Buffett sold his Express Scripts position in the fourth quarter, he likely made a nice profit this time around, too. That's because Express Scripts shares were up 19.8% in the Q4.

A much bigger question mark, however, is the absence of the energy and dividend paying Goliath ExxonMobil (NYSE:XOM) from Buffett's 13F filing.

In a recent interview on CNBC, Buffett admitted to abandoning his $3.68 billion stake. The Oracle of Omaha declared that his decision to sell his ExxonMobil stake was based on the fact that "its current earning power has been diminished significantly from what it was a year ago, as is true with all oil companies" and because "we thought we might have other uses for the money."

Selling his massive stake in ExxonMobil and his smaller stake in ConocoPhilips suggest that Buffett doesn't believe that crude oil prices will recover anytime soon. If so, then investors take-away could be to fade rallies in energy stocks for now.

George Soros
While Buffett is much more willing to own companies for the long haul, billionaire hedge-fund legend George Soros tends to trade stocks more often.

Soros' revolving-door approach means that companies he buys or sells in any given quarter could just as well be sold or bought in the next quarter. However, it's still worth keeping an eye on Soros' market movements given that he's been among the industry's best money managers for decades.

Apple, Inc (NASDAQ:AAPL), Intel, and Stryker Corp were among Soros' biggest sales in the fourth quarter. Exiting the third quarter, Soros had owned $150 million worth of Apple, $133 million worth of Intel, and $130 million worth of Stryker.

Whether Soros' sale of Apple is a bearish bet on Tim Cook's iWatch is anyone's guess, but Apple's absence suggests that at a minimum, Soros thinks it makes more sense to trade around a position in the megacap technology company rather than bet its shares will keep notching new highs.

Regardless, Soros probably still pocketed a nice profit on his Apple stake given that Apple's shares were up by more than 10% in the fourth quarter. Soros may not have been as happy with Intel's shares, which increased by 4.9% last quarter, but he was likely quite happy with how his bet on Stryker played out. After Republicans came out ahead in the November elections, optimism for a repeal of the medical device tax sent Stryker up by more than 17% through year end.

David Einhorn
Einhorn is worth watching for two reasons. First, he turned a $900,000 grub stake into Greenlight Capital, a long-short hedge fund that has more than $7.8 billion under management. Secondly, Einhorn has proven himself to be a savvy investor that has made some stellar investment calls, including his accurate prediction in 2008 that Wall Street's Lehman Brothers was doomed.

Unlike Soros, Einhorn chose to hang onto his Apple stake in the fourth quarter, but he did unwind a slate of other positions.

His biggest sale was the unloading of his $250 million position in health insurer Cigna Corporation (NYSE:CI). He also punted a $124 million stake in American Capital Agency Corp and a $121 million position in National Oilwell Varco.

The Cigna sale is intriguing because it occurred just as Obamacare's second open enrollment period was kicking off. That could mean Einhorn decided Cigna's 17.7% return last year was good enough to book ahead of uncertainty tied to the second open enrollment period and the Supreme Court's decision to hear a pivotal case questioning the validity of Obamacare subsidies.

Einhorn's decision to part ways with American Capital may have similarly been made simply because it was a good time to lock in a solid return. American Capital's shares jumped by 31% last year. However, it's Einhorn's selling of National Oilwell that may have been best. Despite shares falling 6.6% in 2014, collapsing crude prices have forced National Oilwell's shares down by another 16.5% this year.

Tying it together
A long-term approach should be the focus of most investors; however, knowing which stocks to own for the long haul can be tricky. Sometimes, trends change, and investors need to change with them. After all, plenty of long-term buy and holders would have benefited from selling shares in companies like Radio Shack and Eastman-Kodak at some point along the way.

Although the companies these gurus unloaded last quarter aren't likely to end up in the dust bin like those two, their decisions to sell should prompt investors to revisit their reasons for being long and make sure nothing has changed that would suggest selling them, too.

Todd Campbell has no position in any stocks mentioned. Todd owns E.B. Capital Markets, LLC. E.B. Capital's clients may or may not have positions in the companies mentioned.The Motley Fool recommends Apple, Berkshire Hathaway, CVS Health, Express Scripts, Intel, and National Oilwell Varco. The Motley Fool owns shares of Apple, Berkshire Hathaway, Express Scripts, and National Oilwell Varco. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.