I think that there is a special group in Discord for selling and buying. Also there is a topic here, on forum with trade opportunities. XDAG is not yet listed on exchanges, 'cause this new, experimental coin project needs time to develop and mature. But you can still sell coins (I would not suggest you because price will be low still - it's absolutely up to you). If you do not know how to transfer coin you just need to read this forum thread from the beginning.