Give this one a try:
ULTIMATE TEEN COMIC SWAP
Let your teens know which specific day and time the swap will occur and have them bring in comic in during library hours the week prior to the event. For each gently used comic they bring in give the teen a "comic buck" or ticket that they will use to "purchase" comics during the swap. On the day of the program set up the comics spread over the tables and then open to teens. They can use their "comic bucks" to "purchase" gently used comics.
Advertise the event well in advance so the teens know to bring their comics to trade. Open the trading site (couple tables in a room) and let the teens trade comics between themselves. You probably want to stay in the room to mediate some of the trades, but this version is much less staff intensive. :)