Although we don't believe in timing the market or panicking over daily movements, we do like to keep an eye on market changes -- just in case they're material to our investing thesis.

With a slew of economic reports released today, markets finished down modestly, as the Dow Jones Industrial Average (^DJI 0.15%) fell 65 points, or 0.4%. As the drumbeat of earnings reports roll on, stocks reacted to disappointing numbers by Goldman Sachs and Citigroup, as the two major banks saw sharp declines in their bond-trading divisions. Goldman said overall profits fell 21%, and Citigroup said bond trading revenue dipped 15%, as the two stocks fell 2% and 4.3%, respectively.

Economic data was mostly in line with expectations as initial unemployment claims fell modestly to 326,000 from 328,000 the week before. The Philadelphia Fed said its manufacturing activity gauge rose to 9.4 from 6.4 in December, above expectations, but the National Association of Home Builders Housing Market Index slipped from 57 to 56.

Turning to individual stocks, Best Buy (BBY 2.60%) got slammed today, falling 29% after investors freaked following the company's update on holiday sales. The electronics retailers said domestic same-stores sales fell 0.9% during the crucial period, and that operating margins were expected to fall 175 to 185 points in the fourth quarter. The two items were nails on the chalkboard for investors, who had pumped shares of the big-box chain higher as new CEO Hubert Joly's turnaround strategy had appeared to take hold. Overall revenue fell 3%, but online sales jumped 23%, and international comps improved by 0.1%. Joly said that going into the period, "We said that price competitiveness was table stakes and an intensely promotional holiday season is unfolded." Though Best Buy's numbers were far from the worst in retail, investors didn't seem to buy that excuse. Shares seemed overbought before, so today's tumble may not be such a surprise. With and other online retailers continuing to grab share, the company seems to be operating from a competitive disadvantage.

Elsewhere, Nu Skin Enterprises (NUS -1.47%) continued to tumble as investors responded to allegations from the official paper of the Chinese Communist Party that the company is "an illegal pyramid scheme." After shares fell 16% yesterday, the drop continued as the government said it would investigate the company for distributing false information and conducting illegal business. As Nu Skin derived half of its revenue from China in its latest quarterly report, the charges are serious. With the share price cut by 40%, a binary situation has clearly developed with the stock. Exoneration would surely send shares higher, but Nu Skin being forced out of the country would punish the stock even more. Still, considering the stock has already fallen 40% without any necessary effect on profits, investors may be overreacting.