I started building a 1966 set completely of Marked cards. In that vain, I find these cards can be interesting as they can show a good bit of history, creativity, disgust and humor.
I have started a Facebook group for Marked Vintage Sports Cards. The purpose isn't really for making large profits, although you can sell cards on there. If anyone would like to join, here is the link.
https://www.facebook.com/groups/2004927789660039