As I know Magento does not have the functionality of selling items without quantity, but with general weight or volume.
I guess it will be possible to accomplish in this way - first of all you need to allow at your shopping cart settings the usage of decimals. In case you sell products with measurements (weight, length, volume), you can allow customers to purchase a quantity decimal. For example: the product will be 1 L of milk for $1000, but the person will be able to buy 250 ML of it if he enters value 0.25 and the price will be $250, if person wants 500 ML, he indicates value for weight - 0,5, but the price will be $500.
In this way you will be able to track stock of your product, as if the person purchases 250 ML, the general product weight will be 750 ML (1000 ML-250 ML). It may be a bit confusing for your clients, so you need to specify somewhere in the description of the product explanations concerning the determination of the value and instructions how to selection for weight.