I have sold through C24.
There is risk to any purchase not made in person. In my mind, C24 is less risky than most, aside from OF. But even OF has buyers with no history who join only to connect with a seller. I have had that happen a couple timed and it worked out.
A good method for reducing risk is to communicate with the seller and try to determine if they are trustworthy. It's not fool proof, as a swindler is also a good lier. But I don't think most people are master criminals. Find out their history with the watch. Are they a collector or just found the watch in a garage sale? Then it's up to you to determine if they are worth the risk.