If you click
here, you'll find a link to a pdf about pin trading etiquette. While this document happens to talk about Disneyland, you'll find that the same guidelines apply to Disney World as well. You can also find good information on wikipedia.org if you search for an article called "Disney Pin Trading."
While you should be friendly and polite to someone who approaches you about pin trading in the parks, you are under no obligation to trade your pins. As a guest you are fully within your rights to decline to trade a particular pin. (Different rules may apply to cast members.) Some pins are quite valuable while others are not, you should in no way feel like you have to make an inequitable trade. And you should not have to give up a particular pin that sentimental or aesthetic value to you.
I would, however, caution you about wearing your most favorite pins in the parks for another reason ... possible loss. Even if I buy those super-duper screw-on pin backs that are supposed to keep your pins safe on their lanyard, my pins still end up falling off. I've lost more than one this way. So, if you have an absolute favorite that you would never part with, you might want to think twice about wearing it into the parks.
Hope this helps.