My wife has a cousin who runs a farm. Among other things, he sells hay, sometimes in small bales and sometimes in the monster large bales. Most of his customers will show up when he's not around, load their truck, and leave a check. Unfortunately, one or more of them have perhaps been helping themselves to more than they've paid for. I offered to help design a simple surveillance system so he could say "gosh, Bob, I see you took five bales of hay, so you know you owe me $X." The goal here is to help keep honest people honest, not to defeat determined attackers. (That means, for example, that there's no need for night vision.)