You can simply address it to "Property Owner".However, if you're trying to do a more personalized mailing...like a yellow letter, you're going to want to personalize the letters.Also, if this list provider doesn't give you the property owner's name, I'm guessing you aren't being provided a large number of filtering tools either, so you're probably not targeting a specific subset of owners. 

A lot of people don't follow through on anything, so I've learned to filter motivation and abilities a bit.  

Filters are available on Zillow to only show you these properties.  

A voicemail system is great for solving call abundance and filtering however when beginning answering may be your best option. 

It's the only "premium filter" that we pay extra for.  

Just filter those out and maybe plan to follow up in a few weeks.

Just make sure to use the correct filters

I am building a list in ListSource at the moment and am wondering how you filter for properties under median value.

So I would want to learn how to do that first and then set up my filtering system based upon what my personal prospect filters are.

My suggestion is to refine your strategy to filter out these people either via the response mechanism in your marketing or by being more transparent with what you plan to do/offer.