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To reinforce what Tom said, many real estate sites use ActiveX which will only work with Windows and only with Internet Explorer 6 for Windows. The creators of the real estate web packages are blocking out all non-Windows users and all users of alternative Windows browsers such as Firefox and opera.
There is a way to change Safari's user agent, which is the calling card Safari uses to identify itself when it accesses a web site and you can change the user agent to IE/Win 6. This will only change how Safari announces itself and it will not make Safari emulate the browser whose user agent you are using. If you want to test this out for yourself, you need to enable Safari's debug menu either by downloading the free utility
Safari Enhancer or by opening Terminal (in the Utilities folder in Applications) and pasting the following into it, followed by pressing return and closing Terminal:
defaults write com.apple.Safari IncludeDebugMenu 1
Changing the user agent may get you past the front door, if the site uses ActiveX, you will still not be able to actually use the site.