LONDON -- One of Warren Buffett's famous investing sayings is "be fearful when others are greedy and greedy only when others are fearful" -- or, in other words, sell when others are buying and buy when they're selling.

But we might expect Foolish investors to know that, and looking at what Fools have been selling recently might well provide us with some ideas for investments that are past their prime

So, in this series of articles, we're going to look at what customers of The Motley Fool ShareDealing Service have been selling in the past week or so, and what might have made them decide to do so.

The end of blockbusters?
Many people buy GlaxoSmithKline (GSK -0.83%) (GSK -0.83%) for the dividend, and its shares are currently yielding a substantial 5.3%, which is well above the FTSE 100 average. So what might have prompted Fool ShareDealing customers to put the company in the number 4 spot in the latest "Top Ten Sells" list (based on aggregate data from The Motley Fool ShareDealing Service)?

Well, like many pharmaceutical companies, GlaxoSmithKline is under pressure from loss of patent protection and 'exclusivity' on many of its products. It can't now rely on a range of "blockbuster" treatments to reliably generate multi-billion pound annual revenues, and is looking to diversify its R&D pipeline into a broader assortment of "smaller" therapeutic products, while still hoping some of them may also turn into big sellers. Such a change in strategy is obviously not without significant risks.

Furthermore, a report by the IMS Institute for Healthcare Informatics (link opens PDF) suggests that spending on brand-name pharmaceuticals will be increasingly supplanted by generic alternatives over the next few years, to the tune of $120 billion industrywide. GlaxoSmithKline will be increasingly reliant on its "consumer health care" range -- drinks such as Lucozade and Ribena, dental products like Cordasyl mouthwash and Macleans toothpaste, and nicotine replacements like Nicorette -- to make up the shortfall in its revenues. It's far from certain that's possible, which could threaten both its ability to maintain its dividend and its share price overall.

GlaxoSmithKline is also looking to cost-cutting to improve or even just maintain its margins, and to acquisitions to help its organic growth. Reducing costs is never easy in business that relies so heavily on expensive R&D -- cut a bit too much, and it can take many years to recover lost ground, if ever -- and it's easy to spend too much on "buying growth."

So perhaps some investors have fallen out of love with "big pharma" and may now be looking elsewhere for reliable dividends over the long term.

High-quality income shares
One investor who still loves "big pharma" -- including GlaxoSmithKline -- is investing legend Neil Woodford.

Even if you're a seller of GlaxoSmithKline, if you're seeking high-quality income shares, you'll want to get hold of "8 Top Dividend Plays Held By Britain's Super Investor" which reveals his other favorite dividend-paying companies.

Download the free Neil Woodford report today -- but hurry, this report is available for a limited time only.