"I didn't gain anything last turn so there's no point playing Smugglers"
See, the first 2 are a good example of fair bluffing in Dominion... but I'm pretty sure that I would call this one flat-out cheating. What cards someone just gained should be public knowledge, and if playing with Smugglers, then that question should have to be answered honestly. Otherwise Smugglers cannot be legally played correctly.
I suppose you could say that once Smugglers has been played, then you have to honestly answer what cards your opponent gain, but if your opponent is asking before they played the Smugglers "what did you gain last turn," then you're not necessarily obligated to answer honestly or at all. I'm not advocating playing this way, you're being kind of a dick if you do, but I don't see why that would be illegal.
Well the thing is that playing most games IRL, including Dominion, relies upon open communication for the purposes of knowing the gamestate. Players shouldn't be required to carefully watch and track each movement that their opponents make; this would slow games down greatly. Instead, I expect to be able to use communication to find out what my opponent just did.
Like, if he buys a Death Cart, I'd much rather he just honestly answer "which ruins did you get" (if I care to know) than to take the time to grab one, wait until everyone who cares has had a chance to remember what's there, then grab the next. Or in this or any other game, if I want to know how many cards someone has in his hand, I'd rather be able to ask and get an honest answer than to be expected to take the time to make sure he's holding them in such a way that doesn't hide the number, then count them myself. To be more general, IRL gaming is a mostly social experience; people may be chatting about stuff while waiting for a player to take his turn. If he uses his buy while an opponent happened to not be paying attention, then he should be required to tell people what he bought. Perhaps a tournament setting would be different.