There are drop shipping sites..
So Redbubble, I have a Redbubble store, there are mock ups of products on there with the designs I've uploaded to the products they will get printed and ship, which I have selected.
You trundle a long and you want a mug with guineapigs on it, you search and find my Redbubble store, buy the mug with guineapigs on it...
Redbubble email me and tell me you've bought a mug with guineapigs on it. I do a little clap and then read the bit that tells me this earned me 23p.
They tell whoever prints the mug to do that, and to send it to your address.
They email me and tell me they did all that, and what the breakdown of costs was, and at the end of the month they send me all the money I earned.
So somewhere between 23p and a few quid.
I could heavily market my Redbubble store and upload thousands of items, bug the shit out of everyone I know, maximise my SEO etc etc...
But I can't be arsed, its simply there to make a passive income from designs I'd had to produce for other reasons (or just felt like playing about with).
Not all drop shippers work like this though, I could be a drop shipper myself...
So I could advertise that I can get you a product, you ask for it, i go and order it from Etsy and ask them to send it to you.
It means you can run an online store without holding stock. If like me, you have no room for stock or no capital to purchase stock with, then it is brilliant.
The downside of course is that for you as the buyer, it can be harder to work out who is responsible for your order (with official sites like Redbubble thats not an issue, its clearly them!) and how to do a return if you need to.