I actually find nexus devices are a bear to sell. This obviously comes from owning a lot of iphones and those sell within 10 minutes of posting. The two phones I had the hardest time selling were the n5 and the moto x. The moto x I had to sell on swappa b.c I got 0 hits on craigslist.
It's because you are mainly dealing with techies. And techies who want a good deal (from a buyer stand point)
If I was were in market for used nexus 5 16gb I probably would pay more than $275 for it and a used 32gb wouldn't want to pay more than $300 for it.
But techies who buy nexus have their cost basis of $350(or $399) plus shipping plus taxes included. So they will try to say well it cost me $450 for the 32gb nexus 5. And want to sell it for $400.
recently sold mine for more than it costs retail (excluding tax, so technically that's cheaper, and not much more), but looking at the google play store, it frequently has long shipping times. that may be a factor.