If you want to trade your spare band with other MacRumors members, you will have to start a thread in the Marketplace forum, or make a trade with other members offering bands in an existing Marketplace thread.
Mega trade threads will not be allowed.
EDIT: It seems that it may not be an option anyway. Evidence suggests that you do not get two complete bands, just a single band piece with the pin and two pieces with the holes in S/M and M/L to give the full range of sizes.