Registers a new seller, or updates an existing one when a seller with the same external_reference_id already exists for the authenticated merchant. The upsert key is external_reference_id; it can be set on creation but cannot be changed afterwards.

Returns 201 Created on insert and 200 OK on update — distinguish by HTTP status code, not by response body shape.

Seller logos are mirrored rather than linked: submit a publicly reachable https URL in logo_url and Mondu downloads the image and re-hosts it, so seller reads return a Mondu-hosted URL and not the URL you sent. Omitting logo_url on a subsequent upsert keeps the current logo — it is the one optional field this endpoint does not clear when omitted. A logo_url that cannot be fetched, or that is not a PNG or JPEG of at most 5 MB, is rejected with 422 and an error scoped to the logo_url field.