It's great for a big-ticket, low-margin purchase like an MBP, but poor for cheap, high-margin items like cables and accessories. Which is, of course, where they really make money.
They'll price match a laptop to Amazon, but cannot price match a $2 Ethernet patch cable because they're retail space, not a warehouse.
All that said, if what you need is a cable to get your big-ticket item working right now, a lot of us will find the markup on the cheap, high-margin item well worth it.
I'm not sure you save time: That round trip to the store might take an hour. Conversely, you could defer the task that needs that cable until the day after tomorrow, take 2 minutes to buy it on Amazon, and spend the next 48 hours on other stuff.
To me, the hour trip to go to the store is much more burdensome than 2 minutes on my computer plus 48 hours of doing everything else that I have in my life...I run my brain on a round-robin scheduling algorithm, and there are plenty of other tasks that can keep me busy while I wait for that interrupt to return!
I've bought plenty of SD cards online for my raspberry pi and dashcam. They all die and start giving errors within a month or two. Even tho I went of my way to buy name brand, and not the cheapest 4 star cards. :-/
These are some of the most counterfeited items of all, so I'll only buy them from industrial distributors. I figure they are accustomed to vetting their suppliers and have a reputation to protect.
I love the Raspberry Pi, and all the good it's done for computing. I have 3 of them currently. But, I don't want anything SD card driven for my next SBC. They are too volatile for heavy use.
Can confirm the other comment. TV's are very low margin. It's not uncommon for them to sell at a loss, or even severe loss (if you're buying other higher margin items).
Audio isn't typically so bad, but I think it depends on the systems.
The "home theatre in a box" items outside of Bose tend to be pretty low margin in a lot of cases.
Real amp/stereo/surround setups can be pretty decent margin.
My knowledge is derived from a smaller, mid/high-end outfit, so I'm not too sure about Best Buy but I assume the TV's are about the same.
They'll price match a laptop to Amazon, but cannot price match a $2 Ethernet patch cable because they're retail space, not a warehouse.