Keep it always.
Never sell it.
May God forgive me for the M-1s and the 1903s I traded away for more “modern” weaponry.
Why? I have a M1 manufactured by Springfield Armory in 1943, serial # 2117528 that is in really, really good condition that I've been thinking of selling since I never have a chance to shoot it.
The only reason I haven't sold it yet is because I don't know who to take it to in order to get a fair price for it........