There is a hide field feature in the software that will allow you to choose which fields display to logged out users within the listing details page. The smarty extension described here allows you to do something similar for other pages on the site.

We use some code similar to this within the default design but will isolate it here:

				{if not $logged_in}
					{*Logged out code*}
					You are logged out
				{else}
					{*Logged in code*}
					You are logged in
				{/if}

Conversely you can only display something when a user is logged in using this:

				{if $logged_in}
					{*Logged in code*}
					You are logged in
				{/if}

Or just when they are logged out:

				{if not $logged_in}
					{*Logged in code*}
					You are logged out
				{/if}

The above is Smarty code using the "$logged_in" variable we make available in every page of the system