What do you know about 'dirty' bitcoins and which secrets do they hide?
Bitcoin transactions provide pseudonymity, which is not to be confused with anonymity.
This means that there are ways to trace back those coins that were once used to buy ‘dark web goods’.
Sure, not every case is probative but there are examples when exchanges refused to accept assets associated with criminal services or illegal goods, even if the current owner holds only a part of a 'dirty' coin.
The idea of 'dirty' bitcoins is often promoted by bitcoin opponents trying to discourage its adoption.
So, it is always a good idea to buy assets only from trusted parties. Otherwise, you may be running the risk of holding coins you will not be able to sell.