It needs a combination of a lot of personal effort and marketing, and being successful depends on consistency and of course effectiveness.I also know wholesalers who have started some marketing, possibly sending out a round or two of postcards, and then gave up. 

It seems like most people do letters vs postcards in mailings like these? 

Yellow letters, post cards, offer letters, birthday card style, etc... when you get to a point in your business where you start spending consistent money on advertising you need to dedicate some time to understanding at least basic marketing/branding fundamentals.

Send out postcards offering a free home seller's guide.I never liked it but cold-calling does work.

I didn’t send yellow letters I sent yellow hand written postcards) It’s obvious she’s being targeted by investors.

I ordered my postcards off Vistaprint, bought postage stamps and mailed them myself.

3) How would I figure out what is a good percentage of equity for me to send out a postcard to the owner as part of my criteria?

I did some postcards and yellow letters with them at the time but ran out of marketing budget so hung it up.

I.E. letters, post cards, bandit signs, etc... without proper tracking, split testing of copy, headlines, CTA's.

I think most post cards go straight in the trash, but a hand written envelope is usually at least opened.