Cannot Opt-In New Sandbox User to Business Policies - Catch-22

[laserpix]

Hello,
I have a new Sandbox test user that I am unable to opt-in to the Business Policies system. This is preventing all further API development.
I have encountered a catch-22 situation:
The Web UI is Broken: Attempting to access the Business Policies management page on the Sandbox website (e.g., sandbox.ebay.com/ship/prf) results in an incorrect redirect to the live ebay.com production site, making it impossible to opt-in or manage policies via the web.
The API is Blocked: Following the documented recommendation to create the first policy via the API, I have tried to make a POST request to /sell/account/v1/fulfillment_policy.
This fails with a 400 Bad Request error.
The response body is: {"errors":[{"errorId":20403,"domain":"API_ACCOUNT","category":"REQUEST","message":"Invalid .","longMessage":"The seller profile ID is not valid","parameters":[{"name":"text1","value":"The seller is not BP opted in."}]}]}
This demonstrates that the API requires the account to be opted-in, but the only way to opt-in (the web UI) is broken.
I have also confirmed this exact same "not opted in" error occurs when using your official API Explorer tool with its own authenticated token and the pre-filled default JSON payload.
It appears new Sandbox user accounts are in an unrecoverable state where they cannot be enabled for Business Policies. Please advise on how to get this Sandbox user account properly provisioned so we can proceed with API integration.
Thank you.

[laserpix]

I have solved the issue. The key was to use the API Explorer to make an explicit call to the optInToProgram endpoint with the SELLING_POLICY_MANAGEMENT program type. This successfully provisioned the Sandbox user account, after which the createFulfillmentPolicy API calls began working as expected.

[laserpix]

UPDATE 2:

While the optInToProgram call solved the "not opted in" error and allowed me to create policies and offers, I am now definitively blocked at the final publishOffer step.

I have replicated the entire end-to-end flow using the official API Explorer:

1. createOrUpdateInventoryItem -> Success

2. createOffer -> Success

3. publishOffer -> FAILS with a 400 Bad Request and the error: {"message":"Invalid <ShippingPackage>."}

This proves that even with a perfectly valid offer created moments before via the API, the publishOffer endpoint in the Sandbox is broken. It is incorrectly rejecting the package information.

This is a critical, blocking issue that makes it impossible for new developers to test the full listing lifecycle. Please escalate this to your engineering team.

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[developer-support@ebay.com]

Hi @laserpix,

Thank you for your post!

We've reviewed your query, and to provide you with the best possible assistance, we need some additional information. Specifically, we require the API request you've made, the API response you received, and the rlogid found in the response headers for createOrUpdateInventoryItem, createOffer and publishOffer API. In relation to this, we kindly ask you to submit a support ticket through this link: eBay Support Tickets for assistance, which is a complimentary service. This will help us address your issue more effectively.

Additionally, please remember not to share these details here in the forum post, as they may contain confidential information that, if exposed, could lead to security concerns.

Best Regards,
eBay Developer Support