Should I sell items as stacks or smaller amounts?
Solution 1:
It depends on the rarity of the item
Moonlighter prices are influenced by supply and demand (Actually, just by the supply as far as I know), if you sell a lot of items of the same kind, their individual price will decrease.
Common Items
It is fine to sell stacks of low priced common items, because the price drop caused by the increased supply for them is acceptable
Rare Items
High-priced items on the other hand should only be sold individually and possibly not too many of the same kind each day, because their value can decrease significantly.
The key point is variety
The general rule for keeping the prices nice and profitable is to always have a variety of items rather than a lot of items of the same kind. This applies to both stacks of inexpensive items and individual expensive artifacts.