The only reason to keep your low level items is to unlock Bomb. I made the mistake to throw out my oldest items, so it took me longer to do it…:( But once you’ve done it, just trash as much as you want, but keep (and upgrade) S or SS items, they are super precious (and they aren’t as rare as you might think, but don’t overdo the slot). For many items, once you upgraded it once, it will be more valuable (to sell), so do so in case you get a good item and the day comes where you will never use it anymore, which leads to more coins, and thus more upgrades for higher stats!
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