When baking layer cakes, I cut pan liners out of brown paper bags. I lay the pan down on a bag and draw around it, then cut it out. If there is any printing on the bag, put it face down. This way, you never have to grease the pans or worry about the layers sticking and tearing. After cooling 10 minutes in the pans, I run a butter knife around the edges to loosen, then turn the cake over onto my hand, peel off the paper, and turn the cake back over onto a cooling rack.
Not a good idea. Paper bags may have certain bonding agents or insectisides or chemicals that might be harmful. Best to use parchment paper that is made to come in contact with cooked foods.