LONDON -- One of Warren Buffett's famous investing sayings is "be fearful when others are greedy and greedy 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.

A 23-fold return
The share price of ARM Holdings (ARM) (ARMH) shot up nearly 14% last week after the company announced that its first-quarter profits had soared 58%, and that the number of ARM-based chips shipped by its customers had increased by 35% to 2.6 billion.

That seems to have been enough to prompt some shareholders to at least take a quick profit, putting ARM in the No. 3 spot in the latest "Top 10 Sells" list.*

ARM -- which designs microchips for smartphones, cameras, digital TVs and other devices, and licenses them to manufacturers -- has provided stunning returns for its shareholders, having nearly doubled its share price over the past year, and increased it almost 23-fold since late 2002. (Mind you, like that of many tech stocks, it had fallen over 95% in the preceding 2.5 years, and is actually now less than 3% up on its tech-bubble peak of February 2000. But at least it is up.)

ARM's shares have long seemed very expensive by some measures -- on a P/E of around 50 it has to grow its earnings significantly, year after year, well into the future to be worth buying. But over the past decade ARM has delivered spectacular growth and, according to some analysts at least, seems set to continue doing so -- future earnings estimates put the company on a current PEG ratio of less than 1, suggesting that it's still potentially under-valued.

However, that rather assumes that current growth trends in products like smartphones and tablets will continue -- but at some point such markets will mature and saturate -- and there's an increasing risk that a competitor (such as Intel) will disrupt ARM's current quasi-monopolistic dominance.

ARM has provided its shareholders with stupendous rewards over the past decade, but perhaps some of them feel the risk-to-reward ratio has now tipped against them.

5 shares to retire on
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*Based on aggregate data from The Motley Fool ShareDealing Service.