My 18 year old has been pin trading since she was 8, which is when the whole pin trading thing began! Hard to believe this is the 10th year she's been trading. She has a large collection now. We don't buy many on e-Bay - one in a while when she is running low on "traders" we'll buy a few.
At WDW they sell sets and "mystery pouch" pins that are not terribly expensive. With the mystery pouch you don't know what you are getting until you open it. She has her pins divided between "traders" and "keepers". Keepers are limited edition, annual passholder, DVC exclusives, etc. When you start buying that type of pin, it can get expensive pretty quickly.
Her lanyard is Alice in Wonderland themed. She has Cheshire Cats, Alice, the Mad Hatter, the caterpillar, etc. She doesn't trade pins from her lanyard. Those pins all have locking pin backs to prevent loss. She keeps traders in her pocket. She mostly trades with CM's and especially looks for pins in her theme. She'll trade for other interesting pins though and just keep them with her traders until she decides whether to re-trade or keep.
She has been approached by young kids who want to trade. She politely tells them that she doesn't trade the pins on her lanyard but she gets her traders out of her pocket and offers to trade any of those that the child wants. She doesn't care if she gets a lanyard starter pin (which are very common) because she can still re-trade with a CM.
Pin trading is addictive!