It's never a good idea to blindly mimic the buying and selling activity of billionaires. Investors should always make their own decisions, but it can still be useful to track what high-profile billionaire investors are doing. A few of our Foolish contributors have identified three stocks that billionaires have been selling recently. Here are the details.    

Brian Feroldi: Digging through the SEC filings of billionaire investors can be a great way to find promising ideas. One investor I like to check in on every now and then is David Tepper, the billionaire fund manager of Appaloosa Management, a top-notch hedge fund with a great long-term track record. 

A recent filing shows that Tepper's fund recently sold off a 31% chunk of its sizable position in General Motors (NYSE:GM). That move came as a bit of a surprise to me, as General Motors was the fund's top holding. and since the stock has basically traded sideways since the start of the year, I was a surprised to see him sell such a huge position. The move is even more of a head-scratcher when you consider that the company's stock is currently trading at roughly 7 times next year's earnings estimates, offers up a 4% dividend yield, and has the strong tailwind of low gasoline prices at its back.

It's possible Tepper is simply selling off some stock to reinvest the proceeds in other names that he likes more, so I don't think it's wise to dump your position in this company based solely on his selling. Besides, Tepper is probably still very bullish on the company's future, as it remains his fund's top position, but this sell-off is certainly worth noting.

Daniel Miller: Fact: The department-store space is losing market share and is highly exposed to growing e-commerce trends. So it isn't surprising to see David Einhorn's Greenlight Capital finish exiting its position in Macy's (NYSE:M) last quarter. Macy's was a small 0.86% of its U.S. long portfolio stake and was established in the first quarter of 2015 at prices between $62.10 and $67.81. The position was cut by nearly 40% during the second quarter and eliminated during the third quarter.

M Chart

M data by YCharts

It's been a slippery slope for Macy's since this summer, and its third-quarter results only sent the stock tumbling further after it announced that sales fell 5% to $5.9 billion and profit cratered 46% to a meager $118 million during the quarter. In addition to the poor figures, Macy's lowered its full-year guidance for diluted earnings per share from the range of $4.70 to $4.80 down to $4.20 to $4.30.

Because of the shrinking department-store market, Macy's attempt to revive its top-line growth will be focused on expanding its market share, which will probably be a multi-year process, if even achievable. Until then, the company will shed underperforming stores and continue to cut costs, but if consumers switch to discount stores or other discretionary spending during the holidays -- such as electronics, restaurants, or bigger-ticket items such as vehicles -- the holiday season could lead to a crippling fourth quarter for Macy's.

It's not surprising to see Einhorn's Greenlight Capital exit its small position in Macy's, with the near term looking as bleak as it does for the one of the largest U.S. department stores. 

Tim Green: Macy's isn't the only stock David Einhorn has soured on. Einhorn has been touting Micron (NASDAQ:MU), a major manufacturer of DRAM and NAND memory chips, for quite some time. Micron was one of Greenlight's largest holdings at the end of 2014, and as recently as the end of second quarter of this year, Einhorn was optimistic about the company. The stock has tumbled in 2015, losing more than half of its value.

Einhorn changed his tune following the third quarter, slashing Greenlight's position in Micron by 67%. Einhorn admitted that his original thesis -- that consolidation in the DRAM market would lead to higher industry profits -- had some major problems and that Micron is set to earn far less than expected. The company's profits have slumped as oversupply in the DRAM industry has led to rapidly falling prices. Both DRAM and NAND chips are commodities, and both require large capital investments to be made in advance of future demand. If that demand doesn't materialize, prices can fall dramatically.

Micron's guidance for its fiscal first quarter, results for which will be reported later this month, calls for a steep drop in profits. Micron expects its non-GAAP earnings to be between $0.20 and $0.26 per share, down from $0.97 per share during the same period last year. It's unclear exactly when the DRAM market will improve, and it's possible that Micron's results will continue to deteriorate throughout 2016. The last time DRAM oversupply plagued the industry a few years ago, Micron was wildly unprofitable, posting a net loss of over $1 billion in fiscal 2012.

Greenlight remains a Micron shareholder, but its bet on the company is now far smaller than it was earlier this year.

Brian Feroldi has no position in any stocks mentioned. Daniel Miller owns shares of General Motors. Timothy Green has no position in any stocks mentioned. The Motley Fool recommends General Motors. 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.