After a year of steady and quite remarkable gains, fear has crept back into the stock market. Concerns about the U.S. economy have joined emerging market weakness and jitters about the Federal Reserve's stimulus reduction to send the down 6 percent from the high reached Jan. 15. But savvy traders are advising long-term investors that this selloff is presenting a terrific opportunity to buy stocks at a discount.

"If you're a long-term investor, now's the time to be allocating," said Rich Ilczysyzn, senior commodities broker at iiTrader. "I know there's a lot of pension fund capital waiting to be allocated. They may wait for a specific trigger, maybe 5 percent, or maybe 10 percent. But it's not going to give the retail guy a lot of time to jump on. And what's going to happen is, people are going to miss the absolute bottom."

(Read more: The VIX is surging on market fear, but not loathing)

Ilczyszyn says he's using the dip to buy stocks for himself.

"For me, I'm adding because I've got a longer-term horizon. I'm looking out to the next 15, 20 years," he said. "If that's your time frame, you've just got to buy it."

Jim Iuorio of TJM Institutional Services expects the market will fall 8 percent from its recent high, meaning that 2 or so more percentage points of downside. But he says investors are better off buying early than late.

"I actually did call my brother yesterday—he's not a trader, he's an investor—and told him to start buying," Iuorio said. "For investors, it's time to start buying now, and increase buying as we get down closer to 8 percent of a correction."

(Read more: Market strategists: Don't overreact here)

Iuorio points out that over the past year, investors have become convinced that there are no serious risks in stocks. As this perception changes, it is natural for the market to slide. But he doesn't see the current action heralding a more serious downside move.

"Sixteen ninety is my near-term objective in the S&P e-mini March futures, and at that point, I believe it's worth taking a shot on the long side," Iuorio said. "Corrections never feel like corrections when you're in the middle of them."

(Read more: 'Ugly correction' coming: Dennis Gartman)

Brian Stutland of the Stutland Volatility Group recommends buying select stocks and hedging that exposure by keeping 20 percent of one's portfolio in long volatility positions. He thinks that will prove a more effective strategy than simply buying the S&P.

"I do think the correction has already happened," Stutland said. "But it's still going to be really tough for the market to get back to new highs right away."

—By CNBC's Alex Rosenberg. Follow him on Twitter: @CNBCAlex.

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