Each year, investors tend to brace themselves for tricky stock market conditions around this time. Volume tends to be light as many Wall Street bankers and fund managers are away on vacation, which means there is less buying support for dips in the market.

In fact, according to data going all the way back to 1945, September is the worst-performing month of the year for the benchmark S&P 500 index. August is another bad month, historically speaking, and it lived up to that mantle this year, with the index sinking by 1.7%. 

But this seasonal weakness won't last, so now might be a great time for investors to go bargain-hunting. Not all discounted stocks are created equal, though; some companies might look attractively valued but suffer from structural issues that could prevent a recovery when the broader market rebounds. With that in mind, here's one stock to buy now and one to sell.

The stock to buy: Bill.com

Small business owners wear many hats. They have to be salespeople, product experts, and even bookkeepers, which means their time is precious. Bill.com (BILL 3.21%) is a provider of software tools designed to streamline business processes like accounts payable, accounts receivable, and budgeting to put more hours into business owners' pockets.

As of the recent fiscal 2023 fourth quarter (ended June 30), Bill.com served 461,000 business customers across its portfolio of software products. However, the company believes there are over 70 million businesses in its addressable market worldwide, so it has a long runway for growth. Bill.com has built a robust network of partners, which includes over 7,000 accounting firms, and they recommend Bill.com's software to their business clients because it helps them manage their finances more efficiently. 

Bill.com makes money in two primary ways. First, the majority of its revenue is earned through fees when businesses use its platform to make payments. For example, its flagship digital inbox allows customers to aggregate their incoming invoices and pay them with a single click. Second, Bill.com earns subscription revenue for the use of its various software products. 

The former -- payment processing -- could be a blockbuster opportunity for the company over the long term. Businesses completed $266 billion worth of payments on Bill.com's platform during fiscal 2023, but that's a mere fraction of the $125 trillion worth of payments all businesses make around the world each year. 

Despite broader economic challenges, Bill.com continues to grow rapidly. It generated $1.05 billion in revenue during fiscal 2023, which marked a whopping 65% increase compared to fiscal 2022. The company issued cautious guidance for fiscal 2024, suggesting its revenue growth could slow to 23%. But in the past, Bill.com has often increased its forecasts as it moves through the year, so investors should keep an eye out for that. 

Bill.com stock has fallen 13% since the start of August, so this might be a great time to swoop in and buy. 

The stock to sell: Robinhood Markets

Unlike Bill.com, which has an enormous -- and growing -- addressable market, Robinhood Markets (HOOD 4.44%) has consistently lost customers over the last couple of years, and its business model prevents it from venturing into most countries outside America. The company operates one of the most popular investing platforms for young people, especially those in Generation Z who might be exploring the financial markets for the very first time.

But Robinhood's revenue model relies on a concept called payment for order flow, which is banned in every major Western country except the U.S. While traditional stock brokers simply match buyers with sellers and collect a commission, Robinhood charges zero commissions but forwards its customers' orders to market makers who fill them at a slightly worse-than-market price. The market maker pockets the proceeds and pays Robinhood a fee. 

Robinhood's platform achieved peak popularity in the middle of 2021, with 21.3 million monthly active users. Many of those users have steadily dropped off ever since, with just 10.8 million remaining in the recent second quarter of 2023 (ended June 30). The drop is having severe implications for Robinhood's business -- the company generated $193 million in revenue in the second quarter, which was down a whopping 57% from the same quarter two years ago. 

But rising interest rates have been a saving grace for Robinhood recently because the company has $6 billion in cash on its balance sheet, plus $3 billion it's holding for customers. That money is stored in banks, so Robinhood earned $234 million in interest income during Q2, which was up 244% from the same quarter two years ago when interest rates were near the lowest levels in history. 

In short, the high-interest-rate environment is masking the deterioration in Robinhood's core business. However, many experts believe rates will start to come down next year, and if that happens, this company will potentially be left with declining transaction revenue and declining interest revenue. 

It's no surprise Robinhood's stock price has plunged 87% from its all-time high. But with no sign of a recovery in its brokering business, it still doesn't look like a bargain.