I've used the buy/resell trick every now and then. Just at the beginning, to get those two clerics. It works pretty well, but as I've only tried it on QoT, can't really say how good it can be. I'm mostly sitting at full health anyway. But when health does drop, having those clerics nearby is hand. The problem is, when the clerics die, they're gone. Usually they last long enough that I can afford to buy idol for real.
When I see generals with minions on the first few levels, I usually target them first. If they come back, he's got the idol and it's not worth bother, if not, he won't be getting them back.
As exploits go, it's a neat trick that can be helpful, but nothing special. I wasn't even sure if it is an exploit or not, which is why I used it. As for the other buy/summon thing, where you get an army of minions, haven't tried. Sounds like it could take a few towers, but usually at least some DGs have AoE anyway and can just burn them away. Not sure if it's worth the investment or not.