Below is a snapshot of the owners signup page of Owner Direct Spain.   These are the minimum details required to set up an owners account so that you can access the secure part of the website, to list your property and hold your contact details for potential buyers. 

If you are concerned about the security of your personal details -  The site holds your personal details behind your 'username' so only your username will be visable to buyers, your real name will be confidential.  Only include telephone numbers if you are happy for potential buyers to see your telphone numbers. Your email address will remain confidential but buyers will still be able to contact you without seeing your email address.