I wonder if the folks buying items on eBay that are available elsewhere, sometimes for lower prices, are in a financial situation that makes it difficult to buy from brick & mortar stores. Just about every purchase on eBay can be paid for using PayPal. Your PayPal account can be linked directly to your personal checking account. No credit card required. OTOH, some traditional retailers will only take a credit card for on-line purchases. If the buyer doesn't have a credit card, they can't readily buy from that retailer.
Then, there's the possibility that the items we see sold on eBay are being bought by shills just so the seller can show that he's sold lots of stuff on eBay and gotten good seller reviews.
As for on-line retailers, I generally check three sites: R&L Electronics (generally good prices and if they have it in stock, I'll usually get it the next day), GigaParts (always competitive prices, occasional special deals that the other sellers don't offer, and free shipping on many items), and HRO (also competitive prices and free shipping on many items). If it's an item that DX Engineering stocks, I'll check them, too (their prices are generally competitive, they have free shipping for orders over $100, and I'll usually get the item in 1-2 days).