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.
Why? Did you give your credentials or information to them before selling your btc? Or just a codename and your btc address? But how come you have cryptoexchanges that you can use in Bangladesh? If it is totally illegal, they won't allow these cryptoexchanges to operate?
https://www.bitrawr.com/bangladesh