What you might be able to do is convince the agents to offer the XML feeds as a service to their customers. Develop a windows sidebar/google desktop/iGoogle/Yahoo 360 etc widget (or all of them) that these agents can offer to their clients that will let clients see listings from that broker updated almost live. In exchange, you incorporate those listings into your site database. This gives an added value for the extra work the agents would need to do to make the data file exportable as XML.

You could also check with other aggregation sites to find out if they have an XML format that they use to accept data from agents, and match that existing format. This would reduce the workload of agents, allowing them to upload to multiple aggregators using a single feed.

__________________
The best way to learn anything, is to question everything.