It is possible that other people and groups might have other things stockpiled. But it has to be noted that when barter comes up on survival, preparation and survivalist boards invariably one of the first thing mentioned for barter, and the one thing most write about having in quantity, is ammunition.
If everyone has ammunition, and it is not being consumed very rapidly, its value might be quite low.
I'm not so sure about the last thing you said because I highly doubt "everyone" will have all the ammunition they need in a massive SHTF situation. Besides, over the past couple of years, the value of ammo has gone up for me personally, and I have found myself shooting far less. Meanwhile, I have plenty of ammo. There are thousands of others like me.
Imagine a SHTF world where ammo is not being sold in stores. The value of that stuff will rise like crazy, even if some people have all that they need. No matter how much a small circle talks on Internet about stockpiling ammo, there are other normal folks who just don't talk about this stuff, but they have their one or two guns. They may not be stockpiling ammo, but they'll surely want it when SHTF and they can't get any ammo from Wally World.
Anyway, we don't have to guess. Just analyze the price of ammo in the last couple of years, and there wasn't even a massive SHTF situation.
In a SHTF environment where cash has become meaningless, guns are useful, and ammo is not being sold at retail stores, I predict ammo related items will operate as a type of currency, beyond just merely a barter.