Lynne,

It looks like RETS is a standard for the MLS services, I found a DTD there on the RETS standard. Basically, it will depend upon what MLS that your client wants to draw upon. The basic web service for the MLS, such as Realtor.com and Zillow.com, etc. is the API you need to know to communicate with each server. Those web services should be compliant with the RETS standard already. All you have to do is pass and receive the data in a format that they want to see.

As in all development projects, my solution is to find a solution that exists already and then try to use that in my application. The Aircraft building term, "hammer to fit, paint to match" applies nicely to this mode. Sourceforge works good, but I tried out the software that you have linked to above and am not very impressed with it - but, then again, it could be a good place to get started.

The fact is, RETS appears to be a standard protocol. What you do with it is up to you - display it on demand, cache it in a database, etc. Therefore, your conflicting statements about databases being required and not being required make sense.

I don't have any experience with RETS to answer your question, but it seems much like passing information to a credit card processor. Look at the MLS documentation for their API, for the site you need to get information from and that should be a good place to start, at least from what I understand about the situation.

Hope that helps,