This is pretty generic. Standard plot faire. This will be fairly spoiler heavy, as a warning, but I recommend reading this. It'll make for better reading than the manga regardless.
A hero and his harem is summoned, and the MC is the bystander sucked in. And he gets kicked out for being too weak while the "hero" faces the demon king. But turns out the MC has an inventory that lets him see invisible loot drops no one else can pick up, so he gets to be OP. But don't worry, the MC isn't invincible. Well... by all rights he SHOULD BE, but fortunately he has the mental capacity of a 2 year old who was recently dropped headfirst from the second story.
And... that's only a minor exaggeration. Dude carelessly reveals his cheat, forcing him to flee the kingdom. A group of friendly people help him... and he abandons them for no reason for a party he knows nothing about. That party abandons him to bandits, but fortunately he has no items on him due to his item box. What does our MC (which stands for moron character) do? Reveals the existence of his loot bag cheat to the bandits and dumps out everything he had in it.
I practically wanted to drop it then and there. How... utterly stupid. But I stuck with it. And the MC just continues on that path of stupidity. Facing a giant slime made of sewage with a core inside? Fortunately the MC had a giant golem whose arms could easily reach that cove. Unfortunately, the MC is too braindead to consider that and chose to dive head first into the sewage slime. This kind of thing happens time and time again.
I finally dropped it when the MC almost got scammed into buying a sink... that did literally the exact same thing his current sink did, just in a different way. Like, ignoring that they reveal it to be a scam, the MC was about to pay 5 million coins (a HUGE amount given what they establish) for something that is literally just a side-grade to what he currently has. The only reason he didn't is because someone else convinced him he is getting scammed, to which his response was "Am I really so stupid?" To which I wanted to grab his shoulders and violently shake him screaming "YES YOU BELONG IN A MENTAL INSTITUTE YOU DAMN 29 YEAR OLD WITH THE MENTAL CAPACITY OF A 8 YEAR OLD".