Can't 'Convince to Buy' on Food Stall

I'm trying to complete the Market Magnate aspiration; I'm on the second stage where you have to convince 5 sims to buy something from your stall.

I've been trying for ages and my sim is still on 0/5. A lot of the time when I use the convince to buy interaction, the sim I'm trying to convince leaves immediately after the interaction finishes.

My sim has high cooking, all the items are excellent quality and their Charisma isn't bad (level 6).

Is this a bug? Or is it just really difficult?