You are just selling BTC to anyone interested. You are not an investigator on the people you sold BTC to. Just like a money exchanger accepting clients without necessarily investigating whether they are going to use it for crimes or not. How would you know?
I forgot to mention that BTC itself is illegal in our country which is the major reason of my fear. I sometimes sold BTC to unknown guy and that's what I'm fearing of. If it was legal, I would have nothing to fear at all.
Now this poses a little more problem. But since you're probably not involved in the specific crime narrated in the OP, this will just serve as a sort of a warning. The next time around, you should not make transactions involving third parties such as Facebook where some of your personal information are stored. Make deals involving Bitcoin in ways which do not involve even a little information about you.
Selling BTC to unknown people should never be a problem. This is what Bitcoin is all about. You need not know the owners of addresses you are sending Bitcoin to. On your part, make sure you are not using a single address over and over again.