Just my own opinion and whatnot, but I'd never buy electrical
components from eBay unless I could drive to the seller and personally inspect the item(s), and if it's a semiconductor, they've got to be an authorized seller to get my business. Too many counterfeit parts on the market for me to take chances, and it's not worth saving a few pennies or dollars only to wind up with unhappy customers and myself a shitty reputation for cutting corners. That said, I do have a short list of local shops with eBay store fronts; these are places where I know the owners and employees, and the ones I keep doing business with are reputable. Sorry for the digression.
For electrical & electronic parts, I really like Future Electronics, especially for the little oddball parts they (and no one else) carry. Plus they have an office right here in Grand Rapids. Yay! My other primary sources are Mouser, Newark, Digikey, Allied Electronics, and occasionally Arrow. Sometimes you can get lucky and find a cool doodad on Amazon for a decent price, like non-specific lab or office equipment, but again, caveat emptor.
For non-electronic stuff (cases, hardware, raw materials), I have
many sources. I do keep a 'green book' of good American suppliers handy for just about everything I've run across, from aluminum to zirconium, as well as a second 'black book' of suppliers to
never use again.