We ask at Belz what to use to trade,and we were shown metal and enamal pins w/Disney logo no the back. At Belz they were "2000 pins" and they cost $3.99.
At Lake Buena Vista Factory Stores[1 mile from Epcot WDW gate on S R 535] the trading pins were $1.99 The pins at LBVFS were Character Pins,Mickey,Minnie,Pluto,and Donald Duck.
Metal pins are the only trading kind. All Cm's wear a Lanyard with 12 pin and you can trade one pin for one pin. Asking Please and saying Thank-you are part of the ettiquete of pin trading. Pin Carts are at all resorts and in all parks and you can buy pins at them. There are pins that are Limited Edtions[as 1500 Limited Edtion for Opening Day of Millionaire,Play It!] and General to Public pins[bought from carts]
Pins are Park specific,resort specific,holiday specific,and Character specific.
The best way to start to collect what you like! Pin collectors gather by pin carts at schedeled times of the day.
Any Pin Cart CM's can explain more about the art of pin trading!!
But in the mean time go to the pin trading board here at the Dis,they will be happy to help you with questions!!