There are a lot of ways to catch a bitcoin user because it is not really that anonymous as some threads and researches proven it.
Obviously, people tend to sell their bitcoin because:
1. They want to get the equivalent fiat and use it as they pleases.
2. They want to use the fiat that they had sell to buy again bitcoin (well some of the people who wants to have a great profit do this especially when the price of bitcoin dumps)
This is why bitcoin can't be called as the new currency. People, especially billionaires, get in the train when they see profit and then dump it to gain fiat currency. If only people could see its true potential, Bitcoin could reach or even surpass gold in market cap.
Honestly, we need billionaires who are willing to
HODL and not dumping it.