If someone is sitting on a lot of S&D and 25 good price, and I was heading that way with goods on a nice F.A.G. bonus I'd just blow through his "closed borders" and sell anyways.
Chance I might get intercepted but If I am moving fast due to having a lot of horses (and possibly hitting quickmarch) it's gonna be hard unless there is some observant and well armed sentry perfectly placed to intercept me. If he misses, what's he gonna do attack me on the way out? I have no more goods to steal, he can't take my gold, and assuming the trader is decently armed, all Mr. Closed Border is gonna get is an expensive fight, lose hundreds of soldiers and tens of thousands of gold worth of gear just to teach me a moral lesson? The people who do go to all this effort of actively guarding S&D are going to find it very costly in both battle costs and opportunity costs, so let them, they'll fall behind doing it.
I say this to traders: Don't be scared by "closed borders", these threats of hanging you are emptier than they seem. It's very difficult and very costly to intercept and beat a fast moving (horse equipped) and well armed trader. Just keep chasing that S&D, the payoff outweighs the risk especially where some fools are trying to hoard it (and especially S&D in high price good fiefs).