First - read this blog about getting sellers on board to prepare their homes for market: http://activerain.com/blogsview/1138914/okay-miss-smarty-pants-how-do-you-get-your-sellers-on-board-to-get-their-home-ready-for-market-
The gist of the blog is that if you believe in your heart that the repairs/improvements need to be made and you have the human resources (i.e. great service providers) to help get the job done, your sellers will listen. But if you hem and haw and don't act as if it's important, or if you don't have anyone to refer, they won't. Anyway, read it...
As far as spending time and money putting a seminar together and sending out postcards - well - I can't say I'd advise that. I don't want to be a wet umbrella (or however that saying goes!), but this conversation needs to be had one-on-one, face to face with people who RIGHT NOW are thinking of selling their home, and considering hiring you to do it. The majority of people who would get your postcard aren't thinking of selling, so it will be meaningless to them. Your farming materials need to be of interest to the audience, again, the majority of whom aren't thinking of selling right now.
So, you're on the right track in your thinking, but your message needs to be delivered at the right time, to someone who is listening. And so much of whether or not they'll listen to you will come from YOUR body language and tone when you talk to them about preparing their home, as well as your ability to "help" them get it done.
Does this help?