The UESP agrees: "Surprisingly, even Merchantile is affected by fatigue."

It stands to reason, really. If you're really tired, your mind isn't on what you're doing, and you're liable to be taken advantage of.

That's how I look at it, too. I can become pretty incoherent when I'm tired.

"Gasp, pant, pant...how much....pant, gasp.....do you.....gasp.....want for that.......pant, pant......Left Pauldron?" Yeah, that's really going to impress the merchant. If you had to run to get there, then you're obviously desperate, and should be willing to pay accordingly.

Reminds me of the haggling scene in Life of Brian.

So sorry, we're all out of pants today but we're expecting a new shipment tomorrow. Now about that pauldron, I have a reconditioned one here, barely used, on hold for another customer. (eyes shift left to right and in a whisper) Because I like you I'll let you have it today for an extra 50. Deal?