The stock market was going strong this year -- until just recently. A sell-off is happening. And you might be wondering why. Some investors are simply locking in gains. But others are reducing holdings because they fear what might lie ahead for the economy.

Chinese real estate giant China Evergrande Group has warned it might default on debt. Shocks from that would be felt around the world. And right here at home, Treasury Secretary Janet Yellen says the U.S. may not be able to pay its bills as soon as next month unless the debt ceiling is lifted.

All of this sounds grim. But for the long-term investor, tough times are usually times that bring opportunity -- opportunity to buy shares of solid companies at a good price. Let's have a look at three of the best stocks to buy right now.

Two people discuss something as they look at a laptop at home.

Image source: Getty Images.

Novavax

First on my list is Novavax (NVAX 4.91%). Shares of the biotech company have dropped more than 8% since Sept. 1. Yet, good news could be on the horizon. The company might be the next to enter the coronavirus vaccine market. It aims to complete its U.S. regulatory request in the fourth quarter.

So, why the poor share performance? Investors weren't happy when Novavax fell behind in its timeline to bring its vaccine candidate to market. The company initially said it would complete the U.S. regulatory request in June, but raw materials shortages and related logistics issues held things up.

Still, there is plenty of positive news. Data from the phase 3 trial are solid. The company landed a supply agreement with the European Union for as many as 200 million doses. And Novavax predicts that overall, it will bring in billions of dollars in revenue in the coming quarters if the vaccine candidate is authorized.

Novavax also has a flu vaccine candidate, NanoFlu, that last year met all primary endpoints in a phase 3 trial. I'm optimistic about the commercialization of that product down the road. And Novavax recently launched a clinical trial of a combined flu/COVID vaccine candidate. Considering the performance of each vaccine candidate individually, I'm optimistic about the success of them combined. All of this means plenty of potential catalysts for the stock into the future.

Amazon

I like Amazon (AMZN 1.19%) for its dominance in e-commerce. In the last earnings report, the company said it added 50 million members to its Prime membership program over a period of 18 months. And the company's two-year compound annual growth rate is in the range of 25% to 30%. That's higher than the revenue growth rate of 21% prior to the pandemic. Online shopping isn't going away. And Amazon is leading the way.

So, I like Amazon for that business. But I love Amazon for its top position in the world of cloud computing. Why? Amazon Web Services is a major contributor to profit. AWS makes up 54% of Amazon's operating income. And it's likely this will continue. The pandemic marked a turning point for many businesses: They no longer wanted to take care of their own technology infrastructure. So they turned to Amazon. In fact, worldwide cloud computer services are growing. Spending on these services climbed 36% in the second quarter to more than $47 billion, according to Canalys.

Amazon shares have gained less than 4% this year. I don't see this as a permanent slowdown; I see it as an opportunity to get in on this innovative growth company before the stock takes off again.

Starbucks

Today's Starbucks (SBUX -0.17%) is different from the Starbucks of the past. The focus no longer is sitting in a cafe with a group of friends over coffee. During the worst of the pandemic, the company studied the changes in the market and predicted trends that would last. And importantly, Starbucks adapted.

The coffee-shop giant is revamping its store profile in the U.S. For example, it's opening smaller shops dedicated to order pickup in cities. Starbucks also made efforts to open more drive-thrus. This store transformation is about 80% complete. It's clear that the areas it is targeting are key. Drive-thru orders accounted for 47% of transactions in the most recent quarter. And online orders for pickup or delivery represented 26% of transactions.

I also like Starbucks' brand strength. The company added 1 million active members to its Rewards program in the past quarter. And its total of about 24 million active Rewards members represents more than half of spending in U.S. stores. This is even more than before the pandemic.

Starbucks shares have slipped about 4% since the start of the month. At about $112, they're trading well below Wall Street's average 12-month price forecast of $131.26. So any pullback is definitely an opportunity to pour shares of this hot stock into your portfolio.