You actually do go back in time and this person’s discovered something mind blowing that you get to be a part of. You may or may not die, or you may not be the same as you were before.
Shrooms. You likely won’t die and you may not be the same as you were before.
He’s insane and is going to kill you. You will likely die and will not be the same as you were before… because you’ll be dead, or you’ll have killed someone in self defense and, if you’ve never killed before, will be a changed person. …but also if you die and time is a circle then in a way you did go back in time, but also forwards, and nowhere? You’ll be somewhere on the Jeremy Bearimy.
Fourth outcome is he’s not insane enough to kill you, but is also not a time traveler. Just a high energy friend who’s likely manic but also has enough money for ads and can write with good grammar.
I’d look at it the other way. Imagine you’re a murderous psycho who puts that ad out. Someone shows up at your house after PO correspondence done with little to not trail back you. They show up armed. You shoot them in “self defense” and talk about how they had a gun blah blah blah … Accidentally left the door unlocked … Blah blah blah
I see three, maybe four outcomes:
All these options seem to end with “you may not be the same as you were before” either explicitly or implicitly.
“Must bring your own weapons” slightly reduces number three for me.
I’d look at it the other way. Imagine you’re a murderous psycho who puts that ad out. Someone shows up at your house after PO correspondence done with little to not trail back you. They show up armed. You shoot them in “self defense” and talk about how they had a gun blah blah blah … Accidentally left the door unlocked … Blah blah blah