Let's say I'm selling inflatable bouncy houses. Rather than invest in 25 bouncy houses and crossing my fingers that people will want to buy them, I instead put up as basic website as I possibly can. Something like Shopify where it's basically all done for you..
Then I run some AdWords to drive traffic to my website.
This practice can be a little controversial, but I think it's okay. You could basically have the customer go through the process of making the purchase, and then at some point before you take their money, just indicate that it's on back order. If the customer has gone through the time and effort to enter all their information like shipping address, you can be sort of confident that they intended to make the purchase.
You know the sponsored results when you do Google search? That's adwords. Might be called something different now.