Black backround, the eyes of her hat.. looks kinda... shady~
Suwako is a god or curses or something like that, I believe(or at the very least that is one subset of her powers according to Hisoutensoku), so she'd be very effective at coercing/threatening people into buying them. Also, yes, the black background and the squinty-eyed Pyonta as well. Also how she tells them to "buy it" as an imperative, there's a fairly obvious threatening aura coming from her.
Props on Sakuya for being so quick on the uptake. Usually, a lot of these kinds of 4komas end with "Huh!? What's wrong!?" or "Huh? Why do you look so disappointed?"
