Income-seeking investors love buying dividend aristocrats -- stocks that have increased their dividends for more than years -- so when their stock prices fall it's a good idea to look at buying them. As you can see below, automotive and industrial replacement parts distributor Genuine Parts Company (NYSE:GPC) and industrial supply company WW Grainger Inc (NYSE:GWW) have significantly underperformed the S & P 500 so far in 2017, with (NASDAQ:AMZN) playing a big role in both declines. Let's take a closer look at whether these stocks are worth buying right now.

GPC Chart

GPC data by YCharts

Genuine parts company

Based on the company's third-quarter earnings, its automotive group is responsible for around 53% of segment operating profits. Motion industries is next at around 32%, office products is at around 11%, and the rest comes from the electrical/electronic group. Unfortunately, the automotive group, which operates primarily under the National Automotive Parts Association (NAPA) brand name, is facing industry headwinds this year, and possibly an existential threat from Amazon.

A quick look at comparable same store sales at its key rivals shows the impact of weakening consumer demand for automotive replacement parts. For more color, consider that O'Reilly Automotive Inc (NASDAQ:ORLY) was expecting comparable same store sales growth of 3%-5% in its second-quarter, only to report 1.7%.

Same store sales growth slowing for autozone, advance auto parts, o'reilly automotive and genuine parts company

Data source: Company presentations. AutoZone data is for domestic same store sales and is adjusted to the nearest quarter.

O'Reilly's CEO Greg Henslee puts the industry slowdown down to a combination of a second mild winter (warm winters are less damaging to cars), a slowdown in the growth of miles driven, and pressure on lower income consumers. In addition, Henslee drew attention to "depressed new vehicle sales totals during the period from 2008 to 2011." This is an interesting argument because the sweet spot for demand for auto replacement parts is when an automobile reaches seven to eight years in age. If Henslee is right, then industry demand should pick up in the coming years as U.S. vehicle sales increased strongly from 2011-2016.

US Vehicle Sales Chart

US Vehicle Sales data by YCharts

While the arguments above sound reasonable -- they apply to General Parts Company as well -- they don't do much to address fears that Amazon's move into auto parts is eating into the auto parts retailer's lunch. 

Ultimately, if you buy Genuine Parts Company you are taking the view that the slowdown in end demand is merely temporary and that Amazon doesn't present some kind of long-term structural threat to the industry.

Grainger's margin pressure

A quick look at a chart of Grainger compared to fellow industrial supply companies makes for a sorry picture.

GWW Chart

GWW data by YCharts

As with its peers, Grainger's problem can be summed up in one word: "margin." Industrial supply companies like WESCO International and MSC Industrial Direct Co have faced the same issue in 2017. Essentially, end markets are picking up in terms of volume demand, but pricing power is still weak and raw material costs (steel, copper etc) have increased. The result is a squeeze on margin.

GWW Operating Margin (TTM) Chart

GWW Operating Margin (TTM) data by YCharts

However, Amazon has also played a role, as its encroachment into Grainger's marketplace has forced the company to make a strategic decision to cut pricing in order to win market share. On the second-quarter earnings call, CEO DG Macpherson said, "We saw continued volume growth from our strategic pricing initiatives in the United States." However, the 5% increase in volume in the U.S. in the second-quarter was offset by a 4% decline in price, resulting in a 1% increase in sales.

Of course, it's all supposed to be different for the industrial supply companies in 2017. Investors were surely hoping that a cyclical pick-up in end demand from an improving U.S. industrial economy would result in sales growth and margin expansion, but that hasn't been the case so far in 2017, and it's far from clear that Amazon isn't the reason behind the lack of pricing power.

abstract collection of auto parts in the shape of a car

Image source: Getty Images

Are these bargains?

I would argue that Genuine Parts Company is better placed than Grainger to deal with the potential threat from Amazon and other online merchants. The key difference is the importance of timely accessibility and delivery of auto parts. Customers usually require their cars to work immediately, so the brick and mortar retailers -- which frequently replenish products in their stores -- are likely to remain relevant for a long time to come.

On the other hand, the timeliness of delivery is less of an issue for Grainger's customers, and the company's strategic price-cutting is already a sign of competitive pressure from Amazon and others. All told, Grainger's issue looks to be structural, while Genuine Parts Company's weak auto parts sales growth is more of a cyclical issue.

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Lee Samaha owns shares of Advance Auto Parts. The Motley Fool owns shares of and recommends Amazon. The Motley Fool recommends AutoZone. The Motley Fool has a disclosure policy.