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I am in the UK and am having tremendous problems getting the domain name transferred as the Spanish domain name registry say that it is not possible to transfer a .com.es domain name from one registrant to another!
Since I know nothing about what Spain prefers, my suggestions are the obvious ones. Ask the registrar to refer you to the appropriate regulations and, assuming this is an amiable transaction, ask the present domain owner to help you find out what needs to be done.
Frankly I would be pretty careful with .es domain names - I don't know how strictly their rules are enforced - but they certainly seem to be a lot of restrictions - many more than I have seen for other TLDs.
As far as the transfer goes - all the owner should have to do is update the ownership info to your name, address, phone and email and you will be the owner, no "transfer" needed.
The rules of .es names are very strict. You are not generally allowed to register a generic word as a .es domain in either Spanish or English.
It's crazy that they do allow .com.es names to be registered but not transferred to a new owner.
Take control of the DNS, the online account at the registry, the email address used for contact from the registry, etc. Use the domain as if you were the listed WHOIS owner.
Include in your custodianship agreement:
1. You get a perpetual, worldwide, sublicensable, royalty-free (etc.) license to use the domain.
2. You own the domain, but the seller is going to HOLD it for you in their name at no cost (other than you pay the annual registry fees).
3. The seller will immediately transfer the name to you or any party you name at no cost in the event that such a transfer is permitted by the registry (things change, and you want them to transfer to you when it's allowed.)
4. The seller is prohibited from using the domain or anything similar.
5. THe seller will cooperate fully with any required adminstrative duties related to the domain and the registry.
You'll need a competent IP attorney to draft this, someone who understands domains and also Spain.