So, if you have a nano SIM or don't mind cutting one down... it does not matter where you get the phone if you plan to use it on Verizon maybe other than a guy with a suitcase on the street. here's why:
IMEIs for CDMA Carriers are somewhat meaningless in and of the fact that Verizon the only thing they would not let you get a new active SIM to put in the Nexus if it is from another carrier... however if you already have an Active Nano SIM with Verizon then you can simply put it into the Nexus... It is not like T-Mobile or AT&T who exclusively block each other IMEI's... GSM depends on IMEI for activation and CDMA does not. So In other words:
AT&T Bought Nexus 6 to Verizon and Sprint are OK as long as you already have a Active Nano SIM
Same if bought from T-Mobile
AT&T to T-Mobile or Vice-Versa, must be IMEI Unlocked.
It is somewhat sketchy if you do not have the active SIM from Sprint or Verizon.... They may give you one without having a phone at a store if you find someone willing to do it... typically no from what I have read on other sites.
I am personally doing this myself... so I will 100% confirm when I get my AT&T Nexus 6 and try to use it on Verizon... say it will ship by 18th. I will confirm then... I too bought the Droid Turbo. I feel that it is somewhat lacking but can't put my finger on what or why...? Maybe it is because it is designed to work better on Lollipop and it will just take the upgrade to make it a fully realized device... I just don't know. Never had a Nexus Device and this one seems to be the best despite its huge size, plus fully compatible with Verizon. Time will tell and I will happily let everyone know how it is once I get it and how it works on Verizon.
BTW, by the numbers, buying one from AT&T if it actually works to bring it over to Verizon is actually cheaper. $249 on 2 year contract for 32GB model. Keep it 15 days. Never turn it on with the AT&T SIM. Canecel service. Pay $350 ETF... Thats's $600... They may try to charge you 1/2 a months service and activation, but you can easily show no data, no voice, no usage what so ever, so I think you can get most of those charges refunded.. Even so... that will still just take it up to around the retail Motorola / Google price... not a bad deal if you can handle the CSR calls to AT&T and don't want to wait for Play Store and Motorola availability.