You should be able to sign out of one eBay account and then sign back in to the other using your username and password.
It may be useful to keep more than one account. If you do decide to close one, sign into the account you want to close and then use the information in this link:
https://www.ebay.com/help/account/changing-account-settings/closing-account?id=4199
Make sure you do not close the wrong account.
