Dealing with return to sender mail?

[Jack Lived]

Keep seeing my cards coming back to me. Thinking about trying a skip trace for my best leads or something. Maybe phone calls? I send lot of post cards to owners who list their mailing address as the house I want to put under contract. I think that some are abandoned. Suggestions?

I've heard of findtheseller.This one looks new and is a bit cheaper?

mintedinvestments.squarespace.com

Could be a start up?