This issue has been resolved. If you are still receiving the error message - eBay Error 518 : Call usage limit has been reached, please disconnect and reconnect your integration by following these steps:
1) Navigate to Integrations from the left navigation menu, then select eBay.
2) Click on Settings, located on the top right of the Dashboard.
3) Choose Connect from the options on the left.
4) Select Sign in as a different eBay user.
5) Log in to eBay to authorize the connection.
6) Once completed, you will be redirected to Cin7 Omni.

We will continue to monitor our eBay integration.