IDs never really die.
I have one account that remains open although I have not used it in over a decade.
You can't move feedback from one account to another.
You can re-open that old account, just sign in with it. (Assuming you remember the password.)
Sometimes members start new accounts to have a fresh start, for example is malicious (or honest) feedback has poisoned an existing one.
Sometimes members have multiple accounts, for example a buying ID, a posting ID, and a selling ID.
If you are selling it is best to use the ID with the highest feedback.
If you are buying it's pretty well irrelevant since all buyers have 100% positive feedback.