The stock market has been on a tear in 2023, led by the technology sector. The Nasdaq-100 index, which hosts 100 of the largest tech companies listed on the Nasdaq exchange, has soared 35% year to date. 

But we've just entered a seasonally weak period. August and September tend to be the two worst months for the broader market, as many Wall Street fund managers are away for the summer, which results in thin volume and fewer buyers. Right on cue, the Nasdaq-100 has plunged almost 8% since the beginning of August.

Many individual tech stocks are also heading south, and that's opening a window of opportunity for investors. But not all declining stocks are the same. Some genuinely represent a good value, while others are extending previous declines and warrant caution. With that in mind, here's one stock to buy now, and one to sell. 

The stock to buy: Axcelis Technologies

Axcelis Technologies (ACLS 3.06%) is a semiconductor-service company. While it doesn't produce any chips itself, it builds ion implantation equipment, which is critical to the fabrication process. Axcelis' customers manufacture semiconductors in a variety of categories, from processors (CPUs) to memory (DRAM) chips to storage (NAND) chips. 

In the recent second quarter of 2023 (ended June 30), the company told investors it's seeing strong demand in the silicon carbide power device market, which is a growing alternative to typical silicon-based electronics. Silicon carbide leads to lighter, smaller, and more efficient hardware. Axcelis says most of the strength in that segment is coming from producers of chips for the automotive industry, partly as a result of consumers' shift to electric vehicles. 

During Q2, Axcelis generated $274 million in revenue, which was a 23.8% increase year over year, and well above its forecast of $260 million. It prompted the company to increase its full-year revenue guidance by $70 million, to $1.1 billion. But here's the real kicker: Axcelis has a whopping $1.23 billion order backlog, which means it has more than 12 months' worth of sales in the pipeline. 

Additionally, thanks to careful expense management, the company grew its Q2 earnings per share (profit) by 41% to $1.86. 

Given its strong financial results, it's no surprise Axcelis stock has more than doubled in 2023 so far. But in the midst of the August sell-off, it's now trading 20% below its all-time high and its valuation is extremely attractive. Based on Axcelis' $6.21 in trailing 12-month earnings per share, its stock trades at a price-to-earnings (P/E) ratio of just 25.7. 

That's a 15% discount to the Nasdaq-100 index, which trades at a P/E ratio of 30.2. It's also substantially cheaper than leading semiconductor stock Nvidia, which trades at a sky-high P/E ratio of 225. That makes the recent tech sell-off a big opportunity for investors to buy Axcelis stock for the long run.

The stock to sell: DoorDash

DoorDash (DASH 3.12%) is America's leading food delivery platform, but it's on a mission to expand its business into all segments of retail, and even groceries. While that sounds great at face value, there's a glaring problem: DoorDash's food delivery business is still unprofitable. It was even losing money at the height of the pandemic, when its revenue was surging by triple-digit percentages.

Food delivery is a competitive industry with low barriers to entry. Platforms can't really differentiate from one another because they effectively offer the same service, so they instead compete on price, which creates a race to the bottom. Plus, DoorDash's largest operating cost is marketing, and it can't afford to trim that if it wants to maintain its leadership position. 

A competitive price war combined with sticky costs makes it really difficult to generate a profit. In fact, DoorDash made a net loss of $172 million in Q2 2023 (ended June 30), which takes its trailing 12-month net loss to over $1.2 billion.

In June of this year, the company marked its 10-year anniversary by releasing a new-look mobile application. It features food, retail, and groceries, and customers can build separate shopping carts for each category and have products delivered. In his second-quarter investor letter, CEO Tony Xu expressed excitement for this expansion. He said he likes DoorDash's restaurant marketplace, but he wants to build more just like it.

That's the problem. Xu said DoorDash had to spend $1 billion before it generated its first contribution profit in food delivery (similar to gross profit), and given that retail is a much broader category, it could take an even greater investment to really build a presence. Investors might have to endure several years of cash burn -- which could be in the billions of dollars -- while the company scales these new segments. Xu acknowledged investors' concerns that DoorDash might plan to spend money forever, and he said he hopes it's lucky enough to do so, though it probably won't be the case. 

It's clear investors aren't entirely convinced, given DoorDash stock currently trades 68% below its all-time high. Even though the company continues to successfully grow its revenue at a healthy clip, until it can prove it's capable of delivering sustained profitability, its stock is unlikely to reclaim its previous levels.