Yes you can sell your tickets.
If you purchased a ticket and can no longer attend the festival, you can use the official ticket exchange to resell your tickets. To list your tickets for sale on the exchange, sign in to your Front Gate Tickets account at bonnaroo.frontgatetickets.com and under “Your Account” select “Order History” to list your ticket on the Ticketmaster Verified Exchange.
If you are simply selling or giving the ticket to a friend, then it might not be worth the trouble of listing on the exchange. If the friend you are giving it to simply registers the wristband in their name, they will be OK to attend the festival with that wristband. if you want it mailed to them, log in to your frontgate order and change the shipping address to theirs before the tickets ship 4-6 weeks ahead of the event. PLEASE NOTE though that only the original purchaser can pick up the ticket at will call (but most tickets are shipped straight to you, so this shouldn't be an issue for most people)