I would keep it. To my wife’s dismay I tend to keep things. I’m not a hoarder but I never but anything thinking about ever selling it. Yes I have ammo with no gun to fire it.

Sure, I'd sell it. Before Teh Commie Crud hit, I might have given the advice to take it to a gun show.

Only caliber ammo that I have without a firearm for is .22wrf. Rifle I had was stolen in 1982 and have not found a reasonable priced replacement.
Other then that I have never sold any of my guns so I don't have the problem you speak off.

I keep them in case I go shooting with a buddy who shoots those calibers...or I just give them the ammo.

When my FIL passed he had ammo in several misc calibers that he didn’t own guns for... and now I have it.

23 rounds of 44 Special?
17 rounds of 45 Colt?
56 rounds of 25 ACP?

Yes. Always.

Guilty.

I think I even have some ammo for calibers of firearm I have never owned.

Hell, I've got reloading dies for a caliber I don't (and never have) owned. Mere ammo is for amateurs.

I'd keep it on the off chance you end up with another .38, or I'd sell or trade it locally for something more useful. Shipping it would be terribly expensive.

Keep it. Thus when your grandkids are cleaning out your home, they will (1) have a brief laugh during an otherwise unpleasant task and (2) have a deepened understanding of their late grandpa’s peculiar nature.

You should also keep used gift wrapping & foil as well as drained motor oil. You never know when that might come in handy.

not so much....then again, I've got actual firearms I haven't taken to the range for ....uhmmmm, ages.....

ArtieS beat me to it. I have reloading tools for a caliber I no longer own. I never know when a 280 might show up again.

38 spcl? Sure I’d hold onto it. As common as sunshine.

Of course you keep it. Duh.
It's potential TRADING material...

Not to mention the more exotic it is generally the more it goes up in price.
A good example is 303 British ammo. Not too many years ago it used was available surplus at every gun show and easy to get. People took it for granted like 22LR.
Now... it's 75 cents a round.
Anything self-defense or higher end in pretty much any caliber really goes up.

I still have a couple of boxes of 9x18 Makarov, traded the one I owned 25 years ago. Wasn't that easy to find back then, so I held on to the cartridges.

I bought some .22 last year just because and it was cheap at the time. Never owned a gun in that caliber.
Does that count?

I'd give it to a friend that could use it - I remember how good it felt when someone gave me some goodies. I bought some 38 supers by accident and gave them to a guy at my LGS on fathers day. He loved them.

I swore to never buy another 40 caliber pistol. But I have 200 rounds of WWB hollowpoints and about the same of fmjs.

I have 128 rounds of 556 that I do not own a gun for. I’d be happy to trade it for a 6 pack to anyone in south Florida! Serious.

https://i.postimg.cc/NM6qrNH2/20200705-134244.jpg

And nothing is more appreciated when ammo is nonexistent than a friend/relative placing a box of beautiful golden freedom seeds in your hand-