Because a hacker can claim that they bought an account and it would complicate the process of assisting players who really got hacked. Security is of more interest to us than allowing account trading which would compromise security of accounts on so many levels. An account is personal, it belongs to one person and that is the person who created it. No one else. If you play on someone elses account because the person gave it to you or whatsoever, you're going to be playing on it knowing that the owner can take it back anytime he or she wants simply because it's not your account!
Even if you pass your mail to the new owner, there are still chances that you can recover your mail account (depends on the domain and mail provider) and take the account back. We're not going to deal with such situations, we're going to stop them from happening by deleting the account of people who are trying to sell their account (because obviously they don't want their account anymore, and we'll gladly take it from them for free so that we know there won't be a victim buying it and then possibly losing it because you recovered it).
If you want to play on ShadowCores, create your own account and it'll belong to you for as long as you can keep your own mail safe. If you don't want to play on ShadowCores, then just leave your account as-is until you return.