I use amazon for everything I can (It's a good way to get things really cheap. A lot of movies that are $15-20 in stores, I have gotten for as low as $3), But I am not sure about their selection of toy cars.

As for ebay, I have been using it for 4+ years and although I don't use it much anymore, I've never had a problem. Just make sure you check out the Shipping price before bidding (Some of users jack up the shipping to make extra profit), as well as the feedback of the seller.

I used to buy Hot Wheels (and other brands) cars all the time, Lot's of good deals!