the DVLA wouldn't give you that information due to data protection/fraud reasons,i take it you have the last owners address deatails on the v5,why don't you write to him to find out why he sold it etc.maybe if he had the car in 2014 but only acquired the car in 2016 it maybe that it was on finance so technically he wasn't the legal owner of the car until the finance was paid off.
The V5 is the registered keeper - thats not always necessarily the person/company who actually paid for/owns the vehicle. I bought mine on finance and I am the registered keeper. A lease arrangement would be plausible though and then he bought it at the end of the lease.