There are lots of knock offs out there, but as long as the pin says DISNEY on the back, it should be trade-able in the parks. If you buy on Ebay, and want to collect pins, I'd be careful of pins that say Sedesma, or Propins. Most collectors don't want those pins. Also, don't get a lot (25 or more of the same pin) of Kodak pins or 2000 "dancers". Most of the lanyards in WDW already have one of each of those pins on them and therefore, you can't trade those with a CM. The one rule they will stick to is that if the pin is on their lanyard already, they can't take another in trade. Those two pins mentioned specifically must have been produced in the millions for all of them out there!
Another place to check out is dizpins.com - it is an online group of pin collectors/traders. There is a thread there of people selling pins and you will usually get very good traders from the folks there. Half the ones I buy there I end up keeping!
Keep in mind, this is all for fun. Make sure your daughter find the CM's with the green lanyards - those are kid traders only and usually have good stuff on them too!
Guests will sometimes trade with other guests, so look for the ones with lanyards full of pins. Lots of locals in the area love to find new traders and help them out - especially if they are children!