There are a few different ways to approach it. If you are looking for a specific scale, learn who manufactures that particular scale. For example, 1/64 is largely Hot Wheels, Matchbox, Johnny Lighting, and Racing Champions (Ertl). Each has it's own category on eBay.
Looking for a specific color? Type it in your search like "
black Viper ". You will then see the different categories on the left column. Click on "Diecast" and there you go!
These are just a couple examples and I can give you more details if you like. Of course, I have to keep a FEW secrets just for Mo and myself...
Chris