A better way would be to offer them a service so that they would put all of their listings into YOUR site, and from there it will output xml, rss, csv, etc.. so that they can use that for THEIR site.

This will do a few things..

1. make users go to your site a lot more
2. provide a service that they may not have
3. provide an easy to use interface to add/upload their properties into a database (not manually editing code)
4. give other features down the road like google map integration
5. provide an easy and flexible method for users to download their properties via xml, mysql dump, excel, csv, text, etc
6. and most importantly, charge them to make money.