Imei starting with 99 in LG are said to have unlocking issues sometimes IIRC. But it maybe fixed by now.
Get the one with serial number starting with 6 atleast.
Basically, first three digits of S/N in LG indicate its manufacturing date.
E.g. 601 is 6-01,
6 = 2016 (year)
01 = January (month)
Like I got G4 H818P with S/N 604, which means its. 6-04, manufactured in April 2016. So very latest and fresh stock. LG many of the times has quality issues in early production cycle, so getting latest manufactured stock is better. Make sure that your Nexus 5X has no noticeable yellowish tint on display.
Once you know these things, you will figure out that most of the LG G2, G3 etc in market are refurbished and sold as brand new these days. They show 2013/2014 manufacturing date and still sold as brand new. If you lookup their previous history and activation dates, you will find that they are ~2 years old (used), then refurbished and sold as fresh brand new stock. And sometimes they are so messed up that they contradict their own S/N, IMEI number and its related info.