To get the most $ from your M8, unlock it and list it on Amazon or eBay. If you list it on eBay, make sure to ship worldwide. For some reason foreigners are willing to pay good money for unlocked GSM phones. This is what I do with all my old phones.
Not that you care, but my phone history: Samsung SGH-N105, SGH-S105, Sony Ericsson T68i, Nokia 3650, 3220, Motorola SLVR L7, Blackberry Pearl 8100, original iPhone, 3G, 3GS, & 4, Samsung Galaxy SII Skyrocket, SIII, Note 2, HTC One, LG G2 & Pebble watch, Sony Xperia Z2 & LG G Watch (current)