The melt-up rally appears to be back in full swing on Wall Street, with investors pushing the Dow Jones Industrial Average back within 225 points of a new multiyear high. For bulls these rallies may seem like a dream come true. For skeptics like me, they're opportunities to see whether companies trading near their 52-week highs really deserve their current valuations.

Keep in mind that some companies deserve their lofty prices. Take Gammon Gold (NYSE: GRS), for example. The company, a favorite of Motley Fool precious-metals guru Christopher Barker, reported fourth-quarter results this week that showed a 70% year-over-year jump in quarterly earnings per share and a 26% rise in its average realized gold price. It's no wonder Gammon is ascending to new 52-week highs. But some companies potentially deserve a kick in the pants. Here's a look at three companies that could be worth selling.

Caveat emptor
Shareholders of Dollar Thrifty Automotive (NYSE: DTG) have had a truly remarkable run since its lows of March 2009, returning 10,475%! Dollar Thrifty can base this rally largely on improved fundamentals in the car rental sector and the bidding war over the company between Avis Budget (NYSE: CAR) and Hertz Global (NYSE: HTZ) last year. However, investors may want to review Avis' winning bid before clicking the buy button.

Avis agreed to purchase Dollar Thrifty for $45.79 in cash and 0.6543 shares of Avis stock -- valuing the company at $57.41 per share based on yesterday's closing price for Avis. Dollar Thrifty closed yesterday at $63.45, a 10.5% premium over Avis' buyout offer. I'm no math genius, but without an unexpected higher bid, these numbers don't bode well for longs.

Honey, I don't
Diamonds are forever, but gains in Harry Winston (NYSE: HWD) stock may not be. The company reported a robust increase in sales and a return to a quarterly profit, reversing a year-ago loss. But unforeseen events, particularly the Japanese earthquake, could shake the luster right out of this stock.

The company highlighted that Asian demand was one of the key drivers for growth over the past year, which could prove a double-edged sword considering the bearish forecast we witnessed from luxury diamond retailer Tiffany (NYSE: TIF) this week. Much of Harry Winston's growth came from China, but for now, I remain cautious on luxury spending, particularly on large diamond sales, considering the weakness expected from sales coming out of Japan.

Green in the face
Alternative energy generation is bound to be a popular investment after the near-meltdown of the Fukushima Dai'ichi nuclear reactors following the Japan earthquake. Fellow Fool Seth Jayson reminds us, though, that not all green solutions will be the correct answer and that Capstone Turbine (Nasdaq: CPST) may not be the way to play this sector.

Capstone has been burning through investors' money for years. Shareholders have endured numerous secondary offerings and have watched as shareholder equity in the company has gradually dwindled. Capstone's dilemma is that customers seem to prefer larger turbines rather than the many small turbines it manufactures to generate the equivalent amount of energy. It looks to me like an uphill battle that longs will probably not win.

What's your take? Buyer beware or just click the buy button since everything is going up anyway? Share your thoughts in the comments section below and consider supercharging your investing prowess by adding these and your own personalized portfolio of stocks to My Watchlist.

Add Dollar Thrifty Automotive to My Watchlist.

Add Harry Winston Diamond to My Watchlist.

Add Capstone Turbine to My Watchlist.

Fool contributor Sean Williams does not own shares of any companies mentioned in this article. He would like to remind you not to forget about our friends in Japan who could use a helping hand. You can follow him on CAPS under the screen name TMFUltraLong. 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 that never needs to be sold short.