Searching for things in their unlikely places sure is a weird thing. Unless you're in the mode of "Looking for whatever catches my fancy."
While time is of the essence, there should be no hurrying in making the purchase if you are not sure of what, or which to buy. While at one chance you might get lucky at getting the right stuff, at most chances, the random choice can be incorrect.
But that is not even half of the hassle and the expenses. Let us take a very common example. Your car has some problem and for the last two times that you brought it to the dealer for service check, it was the spark plug. Now, who wants to get out and not drive when you've got a really nice car? But then again yours is going to be on the garage again for checkup and repair.
So what now? You head on and search replacement parts online for overnight and show up with a new spark plug. It is almost the perfect quick fix for your disturbed schedule. But oh no it turns out it is not the spark plug and the mechanic tells you it is some other part that you need. Now you want to return the spark plug and get your money back for it. Then again, the freight charge is often more than the cost of the part, and that would multiply your expense.
Or, you can keep the spark plug of course, (not that it is not good to be keeping spares, but that is an unwanted and unneeded item in your well-budgeted list of expenses).
You can however avoid the hassle. Your grade school teacher would love you for remembering what she taught about the 5 steps in doing the scientific way of things: first, state the problem; second, make observations; third, form a hypothesis; fourth, do the experiment; and fifth, draw a conclusion.
If you are non-techie about cars, or is not sure about what really is the best solution for your car problem, it would be best to leave the job to the experts. Certified mechanics have the right technical skills to identify the problem and the proper knowledge and training on how to best deal with it. Just imagine if it were among those hard-to-find auto parts like an ac condenser for an old vehicle! It takes just a little more patience, but it will surely save you a lot more time and money.