*facepalm* Now I feel like an idiot. My Sim had plenty of green left in her Energy bar, or so I thought (I'm a little tyrant who's made her exert herself much worse before), no sleepiness or exhaustion-related moodlets. But I tried again the next day, with Energy maxed, on the merchant's doorstep at exactly 9pm -- and it worked. Huge relief, thank you ClayMask!
And thanks for the heads-up on MasterController too, NonaMena. I didn't need it in this case after all, but it's good to know that the option is there.