That might have been the case before but now they have basically released the exact copy of the TitanX with 0.75 less ram and a better cooler for £500 less, there isnt any reason for the the Titanxp to be more money second hand than a 1080ti
Apart from their extra ROPs Memory and bus width.
Having said that I don't think anyone bought a Titan worrying about their second hand value.
The 1080 Ti is actually good news for Titan users as not only have they had 7 months use out of them already but they are also good for another approximately 12 months before anything faster comes along in the shape of Volta.