His explaining to her what the unsullied were and how many he had to sell was delivered tiresomely like it was any other deal because it was predicated on the belief that it was any other deal, which it was not. When you go to buy a car the dealer does what he does normally. If you follow up his pitch with, "Ok, I'll take the whole lot, and what you have on the assembly line," he'll be taken aback.
And he was put off before he was taken aback because he was operating on the belief that she was full of shit. When he realized she wasn't, he was taken aback, and like I said, I bet he would have been regardless of the dragons. Dumping his whole army, plus the interpreter, plus the untrained slaves to one buyer didn't come across as something that he was used to doing, dragons or not.
So he is a fool. Regardless of any haranguing about the details, all of this is justifying necessary plot machinations via flimsy character motivations. He's a fool, and that is reason enough I guess, but he's not really a character. He is just an object that the plot moves through in an attempt to grant Dany an empowering macguffin.
I definitely wouldn't just call him an object. He's a minor minor character but what we've seen of him is enough to get a sense of what we're dealing with.
No, he was obviously taken aback only when the dragon entered the bargaining table. Before that everything was an annoyance. Watch that scene again. He comments that 'she thinks he can flash her tits and we'll giver her whatever she wants' and when she asks for the in-training unsullied his just sighs and rolls his eyes. To him it's just a bunch of hot air, but when she mentions a dragon he suddenly perks up and is willing to give her everything. And as for her getting Missandei that's the only moment Kraznys seems to show some respect to her as she clearly knows she can sqeeze some more out of the deal in her favor (hence the smirk and slight head nod).
The question is whether or not this plot point (necessary only in that it's completely arbitrary) made any leaps in logic that fall apart when you think about it, which it didn't. When it comes down to it, he's there to sell the unsullied and he underestimated Dany.
You can't think about the purchase of a slave army as buying people. All he was doing was selling a product. The slaver, living in a slaver city, probably never even considered someone turning their army against him. Think of it more like someone who owns a gun store; they never really consider the buyer to suddenly turn and shoot them.
This is at least how I justify it.
More like someone walks into a gun store, and they don't have any money, but they want to trade you three lamborghinis for the store. If you aren't suddenly suspicious about that, I dunno.
Suspicious about what? That one of the lamborghinis will blow up after the deal is done? Even if there was one scene with Kraznys and we couldn't discern anything about him, Rittsy's point is enough for a lack of suspicion to be believable.
You'd be suspicious that someone might want the guns for reasons that would come back to bite you. I doubt you want to be the guy that sold a terrorist a nutjob a bunch of guns. If you are giving over the ancient equivalent of a nuke, you'd probably be a little more cautious.
And, like you said, "When it comes down to it, he's there to sell the unsullied and he underestimated Dany." Which is my point precisely. Like I said, regardless of the details, he is there to serve the plot. He underestimates Dany so she can get her army. Dany needs power? This is the best way for her get it without having to give anything up (also, Dragons!). Renly is in the way of the plot? Well, let's use a one time shadow man to fix that! Want to save someone? Here's a dire wolf! Arya is stuck? Meet my friend Jaqen! Ect, ect. Dany getting her army is just one in a long line of ridiculous jumps this show makes. If only the sorting hat appeared during Ned's execution, he could have pulled out the Sword of Gryffindor and saved the day. Alas.