Send 1 postcard each month to the list for 6 months.

-Buy a list of absentee owners with equity from listsource then use click2mail to send out postcards to them.

I also mail postcards to every house in my target neighborhood once a month.

What service you used, costs, and message on postcard or letter?

As time passes these people stop building and servicing there portfolio and your neededs could really help them.Consider printing small post cards that simply state your wish to obtain seller finance units.  

I never thought of the physical post card though.

I have read that the response rate increases with a follow up postcard.

Like using glossy material, or saving cost by using post cards vs a standard letter, they want to look all pro but forget the #1 goal, conversion! 

I tried doing things the way the "experts" said and had professional postcards designed and printed and sent out 5,000.

You're better served with an online presence rather than wasting money mailing postcards here.