What is the best platform or CMS (Free) that is good to start a shopping website?
It would have about 20 products for sale in the shop.
You can try Free Opensource Shopping Scripts like Joomla, OsCommerce, Zencart, Prestashop etc.
Free script not safe, if you purchase go through wordpress.com, best shopping script..
PrestaShop will be your best if you need some easy management. But if you can handle Magento, it gives you more better options than PrestaShop. You can Google the comparison in detail as Magento vs PrestaShop comparison