Don’t listen to the 2/3rd thing it’s not true each rifle depending on condition and what someone is looking for at the time. Start advertising it at £600 and see where it goes from there. It is true that a tune on a rifle doesn’t always add the value to it but if someone was looking at buying the same rifle and having the tune done then they would pay the £600 as it would save them time and money. It all depends on the right person looking for the rifle at the time. If in no rush stick out for the £600 or keep it. Just remember once sold you will regret selling it so have a good think first as a tuned springer can be better than a pcp and no air bottle needed or leaky seals need replacing