So much for a correction! The broad-based S&P 500 has now traded higher in nine of the past 11 trading sessions in spite of mounting economic data that would suggest anything but optimism. For skeptics like me, that's an opportunity to see whether companies have earned their current valuations.

Keep in mind that some companies deserve their current valuations. Cisco Systems (CSCO 0.36%) is a great example of a company long overdue for a new 52-week high. Networking equipment makers like Cisco are set to benefit in a big way for a capital expenditures boom from wireless service providers that's starting to work its way down the line to infrastructure providers. With Cisco boosting its dividend by another 21% in March and sitting on $31.1 billion in net cash, the sky could be the limit.

Still, other companies might deserve a kick in the pants. Here's a look at three companies that could be worth selling.

No fairytale ending
Fairytale endings work great in the movies, but you rarely see them come to fruition in the real world. Small-cap biopharmaceutical Inovio Pharmaceuticals (INO -5.20%) has seen shares nearly triple since April on the heels of multiple intriguing studies, but will the glass slipper fit over the long term?

Earlier this week, Inovio released data from a study of its universal H7N9 vaccine that demonstrated 100% protection in vaccinated animals. Also, just yesterday, Inovio released the results of a study involving its Cellectra electroporation delivery technology, which improved response rates for its next-generation HIV vaccine to 52% from 7% in terms of CD8 T-cell response rates.

I'm not planning to take a thing away from Inovio here, as these are important and solid results. Shareholders, though, are doing themselves a disservice by ignoring other aspects that I feel make Inovio much more of a risk than these results might indicate.

To begin with, having a vaccine that works in animals is no guarantee it'll be safe and effective in humans. To that end, even getting a vaccine approved for H7N9 doesn't mean it'll be profitable, as governments around the globe would need to buy a lot of the product to make it worthwhile. With a better understanding of how to quarantine infectious diseases nowadays, an H7N9 vaccine may not be a big moneymaker for Inovio. It's also not the only fish in the pond researching for a vaccine, either.

The other aspect to consider here is Inovio's history. Despite being founded 30 years ago, there isn't a whole lot to show for it other than an accumulated deficit of $238.6 million as of its latest quarter. To add to that, it's funded its business with countless dilutive share offerings and warrants, which have increased the number of outstanding shares from approximately 13 million in 2003 to a whopping 156 million as of the first quarter of 2013. Inovio has had shareholders' hopes up before and it's crushed them every time. I'm not sure how this time will be any different, and I'd suggest keeping your distance.

A stock you can sell right now
When in doubt, I can always turn to my tried-and-true TMFULOI metric to point out the market's most overvalued companies. Earlier this month I ran such a screen and Russian search engine giant Yandex (YNDX) popped up near the top of my list.

Yandex has plenty going for it, including dominant market share in Russia, a country with rapid growth potential and a middle class that's seeing their wealth expand. Yandex is already profitable – which is better than some of its peers can say -- and it grew revenue by 36% in its most recent quarter. Yet I see plenty of reasons to believe its next move could be significantly lower.

One factor I mentioned earlier this month is the fluctuation potential of the Russian ruble. Being practically worthless in the early 1990s, the Russian ruble is one of the more volatile currencies around the globe and can negatively impact Yandex's bottom line if it takes another serious dive.

Competition is another big yellow flag for Yandex. Although Yandex and Google (GOOGL -1.35%) do share some partnerships outside of Russia and on its English website, they are fiercely competitive within Russia. Yandex has done a remarkable job of garnering mobile market share in Russia, but I find it unlikely that Google will simply lay down and allow Yandex to reap these rewards without putting up a fight. Google has considerably more cash to throw at product development and has practically written the book on optimizing search profits in the U.S.  

At 26 times cash flow, Yandex looks like it has "avoid" written all over it.

Is this rally about Dun?
Shareholders in Dun & Bradstreet (DNB), a risk management solutions provider that helps businesses better understand their financial risks and optimize their supply chains, are certainly on cloud nine after it hit a new all-time split-adjusted closing high yesterday. But the thing about clouds is that they sometimes produce rain.

Dun & Bradstreet investors should be particularly concerned about the company's recent lack of growth, especially in the wake of slowing growth in China, austerity-induced contraction in some European countries, and the potential for a slowdown in the U.S. once QE3 gets pared back. In the first quarter, Dun & Bradstreet's total revenue on a GAAP basis fell 5% as foreign exchange rates negatively affected its top line.

D&B has been doing its best to mask its lack of growth by paying out consistent dividends and repurchasing its own shares to help drive up its EPS. The reality of the matter is that without global growth, D&B may struggle to gain new clientele, with the past quarter's revenue decline proving this to be true. D&B may not seem particularly expensive at 13 times forward earnings, but with revenue expected to contract fractionally this year and grow by less than 3% next year, it's not encouraging, either.

I'd consider thinking twice about risk-management companies at the point in time.

Foolish roundup
Sometimes it's as simple as shareholders' optimism and hope getting the better of them, and I think that's what we're seeing in all three instances this week.

I'm so confident in my three calls that I plan to make a CAPScall of underperform on each one. The question is: Would you do the same?