For ebay the step list is as follows:
- Login to your eBay account, and under your account name click “Account settings”
- Then on left hand side in account settings click “Site Preferences”
- Under General Preferences, there is a section called “Third-party authorisations”. In here click “Show”
- Tick the “Revoke this authorisation” box beside any applications or software systems you no longer use, and click apply
For amazon you cannot complete the process yourself. You need to go to https://sellercentral.amazon.co.uk/gp/mws/contactus.html open a case, and ask them to revoke access to your current software provider.
The above assumes your account is a UK based account (potentially with links to other markets).
If you have separate accounts for example in Germany, you would need to open a case for each account you have. For Germany, you would need to open the case at https://developer.amazonservices.de