There are different websites online that record all the bitcoin transactions that are occurring at any given moment in time. If you can figure out the exact time and location of a sale, this can very easily lead you to the potential person making the transaction. Every once in a while, though the people performing these transactions will attempt to hide their identity. To do this, there are a few different tools that criminals will try to utilize
These criminals that try to use bitcoin exchanges to purchase illegal items or services have some tools that they use to attempt to conceal their identity. They use things such as a proxy or VPN service that both do similar things in different ways, trying to confuse anyone that may look into where the transactions were coming from on the globe. The proxy is running through another IP address before it accesses the webpage to disguise where the request for the service is located. A VPN will encrypt the data that the individual is accessing from the web and keeps anybody from seeing what you are viewing as to keep your viewing experience private.
Another way that a buyer and seller can communicate is through the blockchain by micro-messages through a bitcoin transaction. These micro-messages are not necessarily a good way of communicating as they cost bitcoin to send back and forth, but the information left behind in these messages is there for a long time.
Criminals believe that if they are using cryptocurrency transactions that they are 100% safe and secure from getting caught in the act of illegal purchases. What they do not realize is that there are multiple things such as a bitcoin wallet address and the IP address that is recorded along with these transactions can often lead investigators to a source of information. Even if the criminal uses a VPN or proxy, there are still clues that can lead back to the original criminal that performed the illegal transaction.