I don't store my Yahoo passwords on my computer...Be using my computer
Any number of hackers could very easily get all your passwords with a key logger program installed on your computer without your knowledge. Similarly, taking over your computer remotely is fairly trivial for a good hacker.
Again, what YOU do is of little interest to companies- it's what all the other small (and large) businesses that may use their services that companies have to protect against.
And if you think that everyone else in the world is just as trustworthy as you, why even bother with credit cards? Ship your products to whoever asks and have them pay you on the honor system. :)