But why would you risk to leave a giant sum on this old address?
The seller could still have the private key.
One reason are threads / speculations like these, where people analyze old accounts. So you don't freak out masses with giant moves. In some cases, where you trade to people / party that have mutual trust, you can afford not to reveal any transaction to public.
True!
Imagine what would happen if suddenly some outputs from the first thousand blocks started moving?
Half of the forum would await the imminent reappearance of the messias, the other half would start killing each other! :-)
Ente