I recently had a very similar experience: I was trying to use the API to update my billing information (such as first name, last name for example) and I would get a Account does not exist error, which is the same error you are getting.
I was told the billing endpoints are intended to be used exclusively by “master-subaccount partners” and my account does not have subaccounts which explains why I was getting this error. My guess is that your company is NOT setup as a master-subaccount partner either. You said you are setup as a reseller, and it’s probably not a master-subaccount partner. Just to be clear: this is a guess on my part based on what I experienced a few weeks ago.
The wording of the error message is misleading, confusing, hides the actual underlying problem and in my case it sent me on a wild goose chase to figure out why my account did not exist. It would be much more helpful to get a more accurate error message similar to: You are not authorized to use the billing endpoints, or something to that effect.