My first (well, only) Arduino I own came from Sparkfun; I’ve also ordered other components from them as well. I’ve never had a problem, and their shipping was insanely quick - but it helps being only a state away!
Yes, they are hobbyists, but they obviously have some business savvy. According to their history, SparkFun was started because they were in college/university (MIT?) and needed parts for a project - a lot of parts. Nobody could source the parts, so they did it themselves, and a company was born to do the same (and more) for others.
Can you get their stuff cheaper elsewhere? Certainly. Are they perfect on their shipping and customer support? Not likely - who is? Will they bend over backwards to help you? Maybe - how nicely did you ask?
Honestly, for them to be doing as well as they are in this economy says something about them as a business; what exactly that is, I am not certain, but they are doing something right, I think.
Businesses don’t exist to sell things to customers at the cheapest price - they exist to sell things to customers for what the customer will pay for it. Indeed, there have been businesses which didn’t last long precisely because they under-priced their product. Yeah, I know that seems crazy, but we are talking about humans here, which isn’t exactly the most rational species on the planet.
As with everything, you need to shop around, and make decisions based on price, shipping time, needs, reputation, customer service, quality of product, etc. For some things, for me, I would probably buy from SparkFun. For other items, I would go to other vendors - even Ebay vendors (I have a few on my list that I like). For still other items, especially if I need it quick, I will check local stores and surplus outlets.
Finally - sometimes “cheapest” isn’t “better”; sure, you can get a cheap alternator from AutoZone with a lifetime full-replacement warantee; but you’ll be taking that thing in and out of your car every six months - but it must be better because its cheaper, right? What’s your time worth? For me, I’d rather spend the extra money at Napa.