The other thing to bear in mind is that turnover is more important than profit on an individual item, especially if the shop is inundated with stuff, as many of them are.
Keeping something in the shop for months at a price so high that no-one wants it, is counter-productive.
It's better for the charity to sell a lot of things cheaply than very few things more expensively.
If someone buys something cheaply and then re-sells it, so what? The charity have gained something from the sale, and made space in the shop to sell something else.