The auction screen is very badly designed:
Lowest current auction price for a single onion: 3₡ How many onions do you want to sell? / 36 For what price? When reading this you would think that the price you are specifying is the price per unit because the example price is the price per unit. But in fact it's the price for all your items. This means that if you're selling (say) 17 items you have to multiply your desired price by 17 to get the total, and plug that in. I don't know how many times I've wrongly posted the item price rather than the total, but it's a pain to do the calculations even if you don't make a mistake.