That stinks man. I hope next time you'll go buy from guys you know a bit better or take a friend with you. At least nothing hurts anymore, they could've messed you up much worse.
Hmm... I generally keep something on me (knife, palmer, etc) when I leave my room... Don't like to be unprotected. Yeah, man, try and keep a friend or two with you. Oh, and make sure that at least one of you knows how to fight, has been in fights before, etc... Yeah, getting jacked sucks. I've been mugged twice, actually, once alone in a similar situation, once with a friend, just a random asshole. Never really lost much, though. My money's generally not in my wallet, and the other stuff I carry isn't worth much...
I would never carry weapons on me... and would never use them even if I did carry them. No need to paranoia.
Yeah, I would never have weapons on me, and if I did, I could neeeeeeeeevvvvvvver stab someone or shot them or whatever.
Sometimes you gotta do what you have to do. It's not like you should grab a knife every day and say to yourself, "If someone attacks me, I'm going to stab them." But if you're in a situation where stabbing someone is required for your own well being, and you don't have something to do it with, you're pretty much screwed.
A good knock on the nose with a fist is effective enough if it's needed, I see no reason to stab someone who attacks me. I also have a scary warcry if neccesary.