First, read the English article below.

Do not buy fancy, scented, coloured or flavoured salts; the salt needs to be as pure as possible. Make sure it's iodine-free and lacks preservatives -- if it's anything but salt it could irritate your skin/nasal lining / whatever you're using it for. 

Now, please write a short abstract for it in Vietnamese. Abstract:
Mua muối ăn thông thường hoặc muối biển.