For Parts and other car bits, I tend to check between a couple places, my local parts shop, and a few online places, maybe a wrecker if its not a wearing part!. if I have time to wait, (ie not an urgent repair) I will usually go online, as price + shipping cant be beat!
if its a 'consumable' such as fuel, oil or tires (and beer!) I go for quality, price and convenience (not going to drive a mile to save a dollar!)
for tools and such, I buy the best I can afford. usually with the prospect of using them again. in the case of wrenches and socket, I only purchase ones with a lifetime warranty and that are available locally for me. (ie I don't have to send in my old broken one, wait 2 weeks. then pick up the new shipped one at a service depot or other inconvenient location.) I use my tools almost daily! not every one, (save my 3/8 drive socket and 10mm deep socket!! ) But I like to have good tools that I can count on! (not that cheap socket that fails and causes me to break my nose, because I ended up punching my face when it breaks!!) don't ask how I know this!!
I do value customer service, it gets me in the door, if the prices are good/reasonable, I stay and purchase what I need! I understand that my parts guy needs to make some money too!

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2003 X5 3.0i (02/2003) sport package, Sapphire black metallic / Black Dakota Sport seats. compass mirror retrofit, trailer hitch retrofit, Prodigy P2 trailer brake controller installed, PDC retrofit, Lumbar support retrofit. heated rear seats retrofitted. Running boards retrofitted and subwoofer install in progress. 4.8is Brakes installed front and back. Bluetooth TCU installed, Rear Fog lights installed. Retrofitted reclining rear seat backs.