It's a bit random what gets "sold" the next day. It seems normal for your goods to take a few days longer to sell sometimes. The longest I've gotten was three days waiting on a stack of turnips to sell, but that was in high to normal demand for large stack of turnips I rp bought from Aldric and flipped. I think the tutorial says the more in demand the item is, (from the market board arrows) the faster it sells, but I haven't looked at the tutorial in a long while.
This is just something I passively observed while playing casually
Waiting a week in game and only getting one sold. That does seem a bit off, unless the price/demand are low to begin with and stayed low for most of that week, resulting in only one being sold. In which case that's some crazy bad luck, but I ultimately don't know.
That's my 2 cents on it anyway.