It is a bit of a chicken and egg problem. It's been a while since I started from scratch with a new channel with products to sell, so I'm not sure where you start these days and I don't want to lead you down the wrong path. However, if you're trying to "getPurchases" those purchases have to be associated with a channel. I can see it failing if you have a new channel because there wouldn't be any way for there to be purchases associated with it. If you don't already have a channel with products attached I would think you'd need to create a channel, create the products, attach the products to the new channel, and then do the "use for billing testing" thing. If you have any existing channels with products attached try setting one of them to "use for billing testing" and see if getPurchases in your dev channel works.
If I get a chance I'll try turning off "use for billing testing" on all of my channels to see what happens if I try getPurchases in a dev channel.