I'm a little uncertain as to what you're trying to achieve, but if my understanding is correct, you want a form for your administrator to enter estate agents details. Once entered, you then want to display this information on your website.
If that's correct, you need to create a database, a form in your administration area and a php call on your website page.
Use the form for your administrator to enter the details which will be written to the database and then use the php call to render the agents details.
By using unique ID's for each agent, you can create a field (php call) on your website page (only visible to your administrator) to enter the agents unique ID. This in turn will render the appropriate details, formatted in whatever way you wish via css.
The database can contain as many fields as you like, including profiles with unique ID's but tied to a particular agents ID, thus by using a php call, anything tied to the agents ID would be rendered.
It's pretty similar to how Wordpress or some of the more simple e-commerce sites work.
I would be very wary about letting users have direct access. Let them post a profile by all means, but subject to it being moderated by your administrator prior to being published.
Frankly, you're talking about some potentially fairly complex coding and it may be better to either drop the idea, or move to a content management system, where much of this functionality is built in.
Joomla seems to be a favourite amongst estate agents, though I personally have never used it.