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 an indication of investments that may be 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.

So much uncertainty
The big banks were definitely on the minds of Motley Fool ShareDealing customers last week -- some were selling Lloyds, others buying Barclays. Yet more put Royal Bank of Scotland Group (NWG 1.25%) (NWG 1.43%) into the No. 8 position in our latest "Top Ten Sells" list*.

Like all of the major banks, RBS's share price has grown dramatically over the past couple of years -- it's up 60% over the past 18 months, and nearly 50% since this time last year. However, RBS really hasn't had a good 2013. Even before last night's news that CEO Stephen Hester is to step down later this year, its share price had barely moved from its end-of-2012 close. And following the news of Hester's departure, which has resulted in a 6% drop so far this morning, it's actually down on the year so far.

If RBS's recent lackluster performance wasn't enough reason to sell, there's also the fact that, as taxpayers, we all already own most of the company through the government's 82% banking-crisis bail-out shareholding. Sadly, we're collectively nursing a huge loss -- approaching £30 billion -- at the moment. And while the government must be eager to reclaim its -- well, actually, our -- money, there's still considerable uncertainty about when, and how, the government will dispose of its stake, although things may become a little clearer when George Osborne delivers his Mansion House speech next Wednesday (19 June).

A healthy bank should trade at a premium to its net asset value (NAV) -- HSBC is currently the only one of the Big Four to do so. RBS is currently trading at a whopping 70% discount to its NAV -- far, far greater than rivals Lloyds and Barclays. Whilst, in theory that could mean RBS is the bargain of the century, it could also mean the market sees a great deal of risk ahead for the bank.

And with so much uncertainty surrounding RBS -- Will it be split up into separate retail and investment banking companies? What will happen to the government's holding, and when? Will any more mis-selling scandals emerge? Will its restructuring programme, originally supposed to be completed this year, actually be finished in 2014? When will dividends recommence? Who will replace Stephen Hester? -- the market may well be right.

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*Based on aggregate data from The Motley Fool ShareDealing Service.