To keep new items at the end/beginning of the list: Settings — List — Insert new item at the end of the list (toggle it on/off, depending on your preference).
You can then re-arrange items by drag & drop. Simply hold the = symbol next to the item you would like to move and drag it to the desired position.
If you’d rather have your items listed alphabetically, you can, of course, opt for the automatic alphabetical sorting: Settings — List — Keep order always alphabetically.
If you wish to move the already bought items out of the way so you can focus on what you need to buy, go to Settings — List — Crossed off to the end of the list. Don’t worry, when you tap these items again to reactivate, they will resume their original spot on your list.