You can take a picture of your filled out ballot with a cellphone. I don't see how there isn't already a very wide open door for coercion/vote purchasing. It's not 100% fool proof, but you don't need that to sway an election, you just need enough people to follow through.

But even with that, it seems like the better course of action is criminalizing such coercion (which is already illegal). Anyone doing something like that on a scale large enough to matter would be caught rather quickly.