Although I am not a law enforcement officer, I think you handled the situation perfectly.
I know the area that you were in VERY well and trust me ... you were in grave danger! I was an OTR Truck Driver for 15 years and Detroit was one of my regular routes for 5 years. The criminals there DO NOT mess around and are extremely dangerous. That is why when I had to stay the night or the weekend in the area, I always stopped at a T/S is Lima, OH ... it was much safer and only a 30 min drive from Detroit. But this is off-topic so I will focus here. For your own peace of mind, I just wanted you to realize that you were indeed in danger, IMHO. I have personally seen some outlandish violence in that area many times before. It isn't considered "the murder capitol of the nation" for nothing.
I agree with what others have said here. He was acting in a suspicious manner, he continued to advance despite your warnings to him, he commented, "I just want to take it for a ride" indicating to me that he meant to do it whether you approved or not and he kept one hand out of site which tells ME that he either DID have a weapon concealed or at least wanted you to think he did. Then, after it was over and he ran-away, you gave the clerk your card and told him to give it to police, something that I personally wouldn't have done. The clerk was probably "buds" with the guy.
Carrying a concealed weapon carries a tremendous responsibility and knowing when to "pull-it" and use it is not always a clear matter. Personally, I do not carry very often for that reason. IMO, having some street-smarts in this situation could have saved you a lot of grief.
1) NEVER stop and exit your vehicle in a questionable area.
2) Never engage in conversation with strangers in a questionable area.
3) If a questionable individual does approach you, hope for the best but prepare for the worst and stand your ground. (Which appears to be exactly what you did).
"The stupider people think you are, the more surprised they are when you kill them."