Alternatively to selling things in stacks of one, check out what recipes your item is used in and sell things in the smallest amount needed for a recipe that material is commonly used for. Say, for example, you've got some kind of ore that you need 3 of to make an ingot. Sell them in multiples of three.
Just don't sell them in huge stacks, especially in stacks that aren't even divisible by the amount buyers will actually need them in. Buying individual items in stacks of one can be time-consuming (as well as taking up a ton of space on your retainers), especially if there aren't enough stacks of one to match what you need and you end up needing to buy more than you need anyway. So for another example, imagine for this ore I need three of, at the lowest price there's 1, 1, and 5. But there's a stack of 3 at the second-lowest price that would still be cheaper than me buying the stack of 5. I'll usually go for the stack that I need instead of buying that larger but cheaper-priced stack and reselling what I don't use because that's more time- and space- consuming. So take a little time to research what you're selling and see how many are already for sale, and in what stack size.
Just don't sell them in huge stacks, especially in stacks that aren't even divisible by the amount buyers will actually need them in. Buying individual items in stacks of one can be time-consuming (as well as taking up a ton of space on your retainers), especially if there aren't enough stacks of one to match what you need and you end up needing to buy more than you need anyway. So for another example, imagine for this ore I need three of, at the lowest price there's 1, 1, and 5. But there's a stack of 3 at the second-lowest price that would still be cheaper than me buying the stack of 5. I'll usually go for the stack that I need instead of buying that larger but cheaper-priced stack and reselling what I don't use because that's more time- and space- consuming. So take a little time to research what you're selling and see how many are already for sale, and in what stack size.