My 1st thought's about your situation are (you might not like them), wait a while, save the extra £70 - £100 & go for a new Carrera TdF or Triban 3.
Stick some of those semi road type tyres (whatever they are called) on your Mtb in the meantime & lock out your fork (if it has one).
My reasons are simple (like me).
1. You will know it's history (because it's new & has none).
2. You will have some kind of warranty (so you can return it in event of issues)
3. It just feels good.
Sorry if this is of no use whatsoever, just my thoughts.
I am a little unsure you would get £280 for a bike that cost you £300 nearly 3 months ago (sorry) but it is now 2nd hand.