Paste your brand list, one per line. You get the responsible economic operator per brand and a CSV you can map onto the bol.com "Verantwoordelijke marktdeelnemer" or Amazon "Manufacturer contact / EU responsible person" columns of your upload file.
Marketplaces ask for the same three things per product: the operator's name, a postal address and an electronic address (email or contact form). bol.com collects them under "Verantwoordelijke marktdeelnemer" in the product content, Amazon under the manufacturer-contact and responsible-person attributes of the listing, your own webshop wherever the product page shows them. The CSV keeps the fields separate (name, street, postal code, city, country, email, website, phone) so you can concatenate them in the shape each upload wants. Every row carries its source URL and the date it was checked; a marketplace-listing source deserves a confirmation with the operator before you rely on it.
Everything runs in your browser: your brand list is not sent anywhere.