const options = {method: 'GET', headers: {'x-spree-api-key': '<api-key>'}};

fetch('http://{defaultHost}/api/v3/store/sellers/{id}', options)
  .then(res => res.json())
  .then(res => console.log(res))
  .catch(err => console.error(err));

curl --request GET \
  --url http://{defaultHost}/api/v3/store/sellers/{id} \
  --header 'x-spree-api-key: <api-key>'

{
  "id": "sel_UkLWZg9DAJ",
  "name": "Sparks Audio",
  "slug": "sparks-audio",
  "about": "",
  "about_html": "",
  "logo_url": null,
  "square_logo_url": null,
  "cover_photo_url": null
}

{
  "error": {
    "code": "record_not_found",
    "message": "Seller not found"
  }
}