Until the beginning of October, 2018 had been another strong year for stocks, with the S&P 500 up 9% and the Nasdaq having gained 16%. However, the recent sell-off prompted by rising interest rates, trade tensions with China, and fears of an economic slowdown has erased those gains this year and has sparked nervousness that stocks could continue to fall.

Still, plenty of stocks have surged this year after banking gains during the earlier market rally. Investors may be wondering if some of these highfliers are still worth buying, so three of our contributors took a closer look at Exact Sciences Corporation (NASDAQ:EXAS) Shopify (NYSE:SHOP), and Amazon (NASDAQ:AMZN). Keep reading to see if they think they're still buys today.

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A resilient name in healthcare

George Budwell (Exact Sciences Corporation): Most healthcare stocks have gotten absolutely walloped in 2018 due to a slew of geopolitical headwinds. The mid-cap cancer diagnostic specialist Exact Sciences, however, has largely defied this marketwide downturn, with its stock up by a healthy 28% for the year at the time of writing. 

Why is Exact's stock proving to be largely bulletproof in 2018? Two key reasons. First and foremost, Exact signed a co-marketing deal with pharma heavyweight Pfizer earlier this year for its noninvasive colon cancer test, Cologuard. Investors cheered this partnership because it instantly expanded Cologuard's commercial reach by several times. Pfizer also committed a sizable chunk of cash toward Cologuard's promotion, which should boost the test's adoption rate among caregivers. 

Secondly, Exact reported exceptionally strong third-quarter results last month, when it clobbered the Street's consensus revenue estimate by nearly 8%. This outstanding quarterly report, in turn, sparked a number of analysts' upgrades on Exact's shares.

Should investors buy into Wall Street's optimistic outlook? In the short-term view of things, Exact's stock is undoubtedly expensive based on its current price-to-sales ratio of 20.6. That's easily among the highest within its immediate peer group, which may hamper further appreciation in the next few months or so.

On a longer time scale, though, Exact's stock could turn out to be a tremendous bargain at current levels. Cologuard, after all, is only starting to scratch the surface of its massive $14 billion target market. Therefore, this high-flying healthcare stock may indeed still be worth buying -- that is, if you're willing to be patient as Cologuard's commercial launch rounds into shape. 

A fast-growing e-commerce platform

Keith Noonan (Shopify): With recent sell-offs wiping out most of the market's gains year to date, there are suddenly a lot fewer stocks in the 25%-year-to-date gainer category. Shopify is one of the survivors and still trades at roughly 14 times this year's expected sales despite recent volatility. That multiple and the stock's relative resilience compared to other highly growth-dependent tech stocks are testaments to the company's performance and the market's confidence in its outlook.

Shares lost 16% of their value amid the broader market's sell-off in October but are still up roughly 35% year to date. Shopify's valuation might still be too rich for investors without substantial risk tolerance, but those who are willing to weather some volatility could see big returns with the stock.

The company provides customizable online-retail websites to businesses looking to quickly bring their offerings online and have flexibility as they scale their operations. Shopify has been rapidly expanding its customer base and the total merchandise volume conducted through its platform, and there's still plenty of room for growth as more merchant partners join the platform and e-commerce continues to account for a larger piece of the total retail picture.

The company reported third-quarter earnings in October, delivering an unexpected adjusted profit of $0.04 per share. Sales growth also came in ahead of expectations, with a 58% year-over-year increase pushing revenue to $270.1 million for the period, and these strong results likely helped the stock stave off bigger sell-offs amid recent market turbulence. 

If market sentiment takes a more pronounced bearish turn, Shopify's high price-to-sales multiple and lack of consistent profits make it a potential candidate for big sell-offs. However, the company's outlook remains promising, and it has frequently surpassed expectations. E-commerce is primed for massive growth over the long term, and Shopify's position in facilitating that trend makes it a worthwhile stock for growth-focused investors.

A tech giant selling at a discount

Jeremy Bowman (Amazon): Amazon made headlines earlier this year when the company topped the $1 trillion mark in market value, becoming the second company to do so, following Apple a few weeks earlier. However, the sell-off in the tech sector since then has brought both FAANG stocks back under that threshold. In fact, Amazon shares have fallen 26% since then, but thanks to its gains earlier in the year, the stock is still up 30% year to date.

The recent sell-off could present an appealing buying opportunity for investors. Amazon shares fell after its third-quarter earnings report as the company projected slower revenue growth in the key holiday quarter, but the good news for investors is that the company is rapidly ramping up profitability.

With the growth of businesses like Amazon Web Services, its cloud-computing division, its third-party marketplace, and advertising, profits are growing faster than they ever have before and should continue to do so as the company finally reaps the benefits of the network of competitive advantages it's built.  In its most recent quarter, earnings per share jumped from $0.52 to $5.75, and through the first three quarters of the year, it's gone from $2.39 to $14.10, a sign of the momentum the company is building.

Though Amazon stock is still expensive based on conventional measures, trading at a P/E ratio of 85, the company has made a habit of crushing analyst estimates in recent quarters. That means its P/E ratio should continue to fall over the coming quarter, and as it does, the stock is likely to get a boost, especially considering how far it has fallen from its recent high.

John Mackey, CEO of Whole Foods Market, an Amazon subsidiary, is a member of The Motley Fool's board of directors. George Budwell owns shares of Pfizer. Jeremy Bowman owns shares of Amazon and Shopify. Keith Noonan has no position in any of the stocks mentioned. The Motley Fool owns shares of and recommends Amazon, Apple, and Shopify. The Motley Fool has the following options: long January 2020 $150 calls on Apple and short January 2020 $155 calls on Apple. The Motley Fool has a disclosure policy.