I think the thing with cards is... you need a bigger variety... making 10 versions of the same card is not that exciting as seeing 10 very different cards...
So although I only "do" cards and tags... they are all different, only some a very few of the gift tags are similar. the cards are all OOAK ( one of a kind ) and they are priced accordingly since they are all different sizes they all have different price tags.
ETA and I used to teach rubber stamping how to classes in a local store that has since folded. and so, I have a following of people here who know who I am because of it. ( I'm not a demo, never have been I just like to play)
So, I think if you were a card maker I would mix up your card sizes and what you offer before you say cards don't sell. they do. they do very well actually but, not every person will love every card... so do different styles. some CAS some lumpy as all get out. horse trade for images, borrow stamps from your local friends. go to yard sales and discover old stamps that are not on every chain stores shelf in your town
and make sure those people know you made those cards... people always tell me how talented I am to have drawn such varied designs and I always correct them with these were done with rubber stamps... I just like to color.
I think that is a draw for me to. ( I am not making a bunch of die cut with a plug in electronic machine type cards either. )