Search the threads here on the Lee enfields on what to look for traps and pit stops when you find one you like get pics of the rifle hopefully the seller will get pics of the stamping on the barrel near the chamber have to remove the top woods for that its a rare thing to have one in unissued condition or Mummy wrapped and they command a heavy premium the UF No.4's seem to be the most prevelant of these.
If the seller says a rifle is unfired then thats bolocks as they all had to be proofed and sighted for the ammunition so every 303 has had rounds down the tube prior to issue and remember to buy the rifle not the story as some sellers tell tall tales on a rifles past unless they can prove it with provenence beyond a reasonable doubt.
You can get some really nice examples of these weapons but take the time to at the very least learn the basics of what to look for in them as it may save you getting hoodwinked if in doubt post the weapon here like you have and the forum here will always give the best answer they can without handling the weapon.
A couple with the No.4 I always looked at was what size bolt head it had say a 0 or a 1 that is a good start but still I carry a go & no go gauge (Yes I am obsessive) if it has say a No.3 bolt head then I may look a little deeper not saying the rifle is unsafe just the 3 is the largest of the bolt heads, make sure the bolt matches the rifle.
Check the muzzle for dings on the crown or cord wear from the pull through wearing one side of the bore, look down the bore agains a light looking at the lands or a ringed (Bulged barrel) anyway thats just a few of the things I look at or place a WTB add on this site as someone may want to sell one don't know why as once you get one they multiply!
Good luck and Happy new years to all.