I agree. It's foolish to give websites permission to do that kind of thing. There's a very simple solution: don't give them permission. Turn off javascript. Yes, ebay will complain but it'll still work. The power is in your hands. No one is forcing you to use eBay either.
Giving arbitrary websites the ability to run arbitrary code on your machine is just asking for trouble. It's like someone who opens and executes every email attachment they receive. Try out NoScript temp-whitelist only mode that blocks by default and requires manual permission giving.