What you need in order to bid at a Spanish auction
An auction on the Portal de Subastas is open to everyone, but not everyone can take part tomorrow. Bidding in your own name requires three things, and you will not arrange them within the lifetime of a single auction. If you would rather not, or cannot, you can take part through a representative — and then those requirements fall away for you.
Bidding yourself: three requirements
A Spanish tax number. Registering on the portal is possible only with a DNI or a NIE. The portal is unambiguous about this: if your certificate does not identify a natural person by such a number, registration cannot be completed. A non-resident can apply for a NIE, but it takes time — see below.
A digital certificate or Cl@ve. You sign in with a qualified electronic certificate, or with Cl@ve provided it was obtained in person or by means of a certificate. A Cl@ve obtained through an invitation letter is not accepted. The certificate itself generally requires appearing in person.
A Spanish bank account. The deposit can only be made through the portal, from an account you hold or are authorised on at an institution connected to the tax authority's payment gateway. In practice that means a Spanish bank. Accounts requiring several signatures cannot be used. How large the deposit is appears in the notice for the auction in question.
A company cannot register
The portal admits natural persons only. A company therefore always acquires through an individual: at the first bid the portal asks whether you are bidding in your own name or on behalf of a third party, and in the latter case for the company name, the NIF or CIF, the address details and a copy of the power of attorney as a PDF. The award then goes to that party rather than to you. Forget that step and only your own name appears on the bid certificate.
Taking part through a representative
You can have a Spanish lawyer or gestor bid on your behalf. They already hold the certificate, the DNI and the bank account; you supply a power of attorney. That removes the need for a NIE of your own in order to take part in the auction. German- and English-speaking firms in Spain offer this service, usually for a fixed fee per auction plus a success fee.
That is not a detour but a division of labour: the research beforehand — encumbrances, occupancy, costs — you do yourself or with a platform; the bidding and the completion you leave to someone who has the means for it.
How long it takes, and why that matters
You apply for a NIE at a Spanish consulate in your own country, in Spain itself, or through a representative holding a power of attorney in which the application is expressly stated. Consulates themselves tend to quote two weeks to a month; in practice lead times vary and it can take longer. After that come the certificate or Cl@ve, and opening a bank account.
An auction runs for twenty calendar days. So you cannot spot an interesting auction and then get ready in time. Anyone seriously considering bidding in their own name arranges this weeks in advance — even without a concrete opportunity in sight.
What you still need afterwards
Bidding is one step. Registration in the land registry and payment of transfer tax carry their own identification requirements; bear in mind that a Spanish tax number may still be needed there, even if you bid through a representative. Have that confirmed for your own situation before you bid.
This is general information, not legal advice. Conditions differ per auction and per region; always consult the auction notice itself and, where needed, a Spanish lawyer.