Yes, nowadays its being used in crime, they buy and sell online and pay by bitcoin. Because its untraceable. So police cant get clue to caught them. I think its dangerous.
It is not nowadays. Bitcoin has always been used for illegal activities such as the following: Hiring assassins, buying weapons, buying/selling drugs, watching pedophilia porn, and so on. It was, basically, the dark net's currency. And it will continue to be used for those purposes.
And yes, it is even very dangerous to be on the dark net. Who knows what happens to your PC as the hackers could track your IP address. That is why you should be aware of how to secure it.
I agree with you, Bitcoin is decentralized currency it is impossible to trace it now and before it is being used in illegal crimes.Illegal goods are purchased with help of bitcoin and criminals do their business online like what you've mentioned. And Before bitcoin theyuse prepaid cards to commit such crimes. In fact prepaid cards are more anonymous than bitcoin and can be resold without any trace.