[TaoTe]

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Long story short, my buddy left his duffel bag in my car on Friday.

[/ QUOTE ] That's his mistake and not yours. He's not very much of a buddy.
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1) How irresponsible was it that I left his bag out in his apartment hall?

[/ QUOTE ] How irresponsible was it for him to leave his bag in your car, then he blaims you for driving 40 miles out of your way to drop his things, which he left in your car, off? I believe that's called kindness, not irresponsible.
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2) Should I be responsible for all the costs? I honestly don't care about the $$$, but I more care about what's the fair/right thing to do.

[/ QUOTE ] I wouldn't pay anything, and if this person, who you consider a friend, pestered me about the money I'd kick him in the teeth. I have good reasons for this. 1) He left his bag in your car. Apparently he didn't ask you if it was okay to do this. 2)He blames you for getting his bag stolen. You didn't steal the bag, some jerk did. 3) He was unappreciative of you driving 40 miles to help him out. Gas prices! For that alone he is well deserved of a nice kick in the teeth. 4) He's not your friend or buddy. Maybe he's trying to rip you off.

My advice. Don't pay him anything. Kick him in the teeth. Get a new buddy.