Manual Import?

Maybe use 'feeds' ? I've used this method to import thousands of 'store locations" into a custom content type "Locations" (which held things like company, address, GEO Lat/Long, etc)...

Feeds lets you map the csv columns into drupal content fields.... not sure if it would work with entities (what drealty is using). With feeds, you can map the RETS unique id to the feeds import, and it will update only the nodes that need updating, and skip the ones that exist (there are also filters or instructions for feeds on how to handle different situations)

But if you're going to go that route, then you probably don't need drealty, as you can use nodes, instead of entities...

I would also suggest to try Feeds.

In this case you will need Feeds, WSClient and WSClient Feeds - this will allow you to input URL for the endpoint and obtain structured UI presentation for the operations/parameters. I have been using this concept on another project now, not Drealty.

I work directly with a real estate broker firm and doing what you are suggesting would violate most local real estate board policies....there is a reason for the $600 a year access.