There's two options:
1) Allow agents to push data to your portal. This would involve you a) creating a function to process either an XML file or text file b) receiving this file would either require them having ftp access or you providing some sort of webservice ideally SOAP.
2) Get data from the agents. Again this would either involve you downloading data from them invariably it wont' be in the same format so you would need to create a user agent or robot to visit these pages and extract out the key data. Very tricky to get right.
Essentially option one is the only one open to you but as it sounds like there is no industry standard for sending property data you could be the first and this can give you quite a bit of power.
The other gotcha you will need to look out for is the transfer of images as this can mount up to quite a lot of data transfer.
You may also like to keep an eye on this thread here as it is purely for property professionals doing just what you are talking about.
RightMove BLM V3 - Juicy Red Apple