There is a board for collectors here on the dis. Maybe the mod will move the thread there and you'll get a lot more advice. But I'll give ya some.
Ebay:
Please be careful when ordering from the sellers on ebay. There are MANY fake pins that are called "scrappers". Basically Disney has their pins printed in china - cheapest place. BUT the factories that print them often keep the molds of the pins (even though they are supposed to destroy them) and make counterfeit ones from them. They are VERY hard to tell the difference for a newbie, and sometimes even a seasoned pro.
Some (not all) of the people that sell on ebay are buying these fakes directly from china. For pennies each. And selling them to unsuspecting people.
My advise it this. Watch a few sellers for a while. If they keep selling the same pins week after week. Well you gotta ask yourself - how did they get 10 of those pins? And why are they ok with selling them for 75 cents when the rack rate is 7.50?
Fair price,I think, for "rack" (as in not a limited edition) pins is around 3-5 dollars each. Still better than the 6-10 paid in the parks.
Trading:
In Epcot and at DTD there are tables where some pin dealers or Pin Sharks set up. Now Disney does not allow them to "sell" what they have. But they might try and trick you into going into the shop they are set up next to and get you to buy 2 LE pins for the one you want to trade for. And if you're new to trading you may not know that the pin that you want is a common one. Plus those pins they want you to buy are gonna get turned around and sold on ebay themselves. It's a nasty business. Now not all of them are this way. But personally I've read too many horror stories and wouldn't want to risk it.
I mostly trade with CMs, sometimes a fellow trader I see while waiting in line. But again - the CMs will get the fakes on their lanyards too. So have a good look at them. I know I wouldn't want to trade away a 10 dollar pin for a 35 cent one.
Lanyards:
Basic color ones go for around 5 dollars in the store at Disney. But you can get "starter sets" which is a lanyard with around 10 pins from 30-50. I was lucky and got one of the ones they handed out for the million dreams celebration. I like it because it's wider than the standard ones and I can keep more pins on it. Plus I didn't see anyone else with one during my last trip.
All in all trading is fun and very addictive. My intention was not to scare you away from it with all of my above babble. Just wanted to make you aware that there are people out there that will try to take advantage of you AND your kids.

Sad but true...
Anyway - I hope you have a great trip!
