I've got about 530 images on FAA and sell roughly one a month.
I have anywhere from a few to several sales a month on redbubble but make more from FAA as I'm usually selling framed prints there while redbubble tends to be more products.
The POD sites don't do much marketing and some like FAA hardly do any at all, so you need to get your work up on social media to make sales. It's different than stock.