Strong corporate earnings this week continue to propel the S&P 500 to new multiyear highs, with seemingly no end in sight. For bulls, these rallies may seem like dreams 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. Shares of Apple (Nasdaq: AAPL) ticked to new highs this week, as investors applauded the overwhelmingly bullish response to the Verizon (NYSE: VZ) iPhone launch. But some companies potentially deserve a kick in the pants. Here's a look at three companies that could be worth selling.

Green Mountain over-roasted?
Green Mountain Coffee Roasters
(Nasdaq: GMCR) shareholders risk scorching their shares, after the maker of the Keurig single-cup brewing system reported a 67% jump in revenue and boosted guidance last week. Green Mountain hasn't abandoned growing organically, but the company has not been shy about touting its plan to grow through acquisitions, either. However, this strategy will make for some very tough year-over-year comparisons if M&A activity slows down.

Also, more than one-quarter of the company's float is currently held by short-sellers, so this rally could be fueled more by short-covering than actual earnings momentum. Usually, short interest this high is a yellow flag to investors to stay clear. Shareholders may want to heed that warning.

Lulu a lemon?
lululemon athletica
(Nasdaq: LULU) shares have had a nearly uninterrupted rise over the past year, with the athletic apparel maker crushing consensus estimates by an average of 34% during the past four quarters. Just last month, the company raised its own prior guidance after strong Christmas sales. So why would I consider selling here? Because of same-store comparisons, logic, and competition.

I find it very hard to believe that lululemon will be able to come close this year to the 29% same-store sales jump it delivered last quarter. That's strike one. I'm also concerned about just how much residual business the company will get. It has drawn in a solid crowd of affluent first-time buyers, but how much athleticwear does one person need? That's strike two. Finally, Gap (NYSE: GPS) opened a new flagship Athleta retail store in San Francisco last month, with its competing line of athleticwear. That could eat away at lululemon's market share -- strike three.

Buy and it could Hertz
Rental-car company Hertz (NYSE: HTZ) could burn a hole in its tires if this run-up continues. Despite projecting a return to profitability in 2011, the company hasn't earned an annual profit since 2007. I'm also concerned by the $12 billion in debt currently on its balance sheet. Hertz does need to finance deals and purchase new vehicles in order to stay competitive in the car-rental sector, but over the long term, I worry about its ability to meet its interest payments if profits don't come in strong.

Have an opinion on any of these companies? Let's hear about it in the comments section below!

Fool contributor Sean Williams does not own shares in any companies mentioned in this article. He admits to being a Starbucks junkie, and doesn't really know what the inside of a gym looks like. You can follow him on CAPS under the screen name TMFUltraLong. Green Mountain and lululemon athletica are Motley Fool Rule Breakers recommendations. Motley Fool Alpha has opened a short position on Green Mountain. The Fool has written puts on and owns shares of Apple, which is a Motley Fool Stock Advisor selection. The Fool also owns shares of lululemon athetica. 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 which never needs to be sold short.