This question is for the United States in general for online marketplaces.
Some history: Back in 2012, Ebay stopped the sale of spells, and now Etsy has also banned the sale of spells.
They have both banned several things that are not illegal to sell, but questionable.
I am not concerned about whether it is 'moral' or against a religion, but rather whether the seller and/or the site that the spell was sold on could be held liable for anything?
An assumption would be that the buyer would get some proof that the spell was performed. This could be a photo, video, skype session, or some other form of proof. The proof may be downloaded, mailed, or in person delivered.
If it is against the law, what law / regulation? I've seen several posts stating that it's against the law, but no one has mentioned an applicable law.
Also, what would be the legal difference between a spell and a prayer?