You don't enter values of anything. The simplest way to adjust numbers to purchase is to move the slide bar - either click and drag (when you're dealing with large numbers) or just click on the greater than/less than signs at either end. It will increase or decrease the number of units that you want to buy/sell (that is, number of potions, number of lockpicks, etc). The price will adjust accordingly automatically.
Does that help? If not, let's try again.
