Thanks a lot guys.
The domain name is currently with EuroDNS, all they want me to do was start a transfer and ask the buyer to do the same from their end.
I am supposed to receive email from NIC.es(like Ben said above), but the buyer does not have a registrar and have created account with NIC.es.
I asked DAN .com to ask the buyer to create account with EuroDNS which should make it easier but they ask me to ask EuroDNS if they could transfer the domain name to the buyer's NIC handle and this is not EuroDNS process. Although I emailed them to find out if they can do this, still waiting for reply.
Also, I found out that NIC.es can be your registrar too but at a higher cost, I guess similar to Nominet been your registrar.
I have asked DAN to ask the buyer to initiate the transfer from NIC.es and I am waiting to hear back.
Thanks again.