I buy most of my computer related gear at Newegg. I use to buy darn near everything else at Amazon but, I have mostly stopped because of their support of the pedophile books and sexual exploration of young women that recently came to mind.
I used to use eBay a lot. However, I find that often they aren't the bargain they use to be. Especially, when you include the often exorbitant postage/freight charges.
Companies like Crutchfield, B&H, and Adorama which used to be specialty companies now offer more and more products. They all provide good service.
Lastly, I buy a lot of clothing from LL Bean. I used to love Lands End, until Sears bought them up and now charge taxes as well as freight.