Look at the necklace names. You can't enchant it unless it's maxed level ̣(5). The one you get from exchanging Fragments is https://l2wiki.com/classic/Nebula's_Old_Necklace#Lv. 1. Then you have to maxed it to level 5 to be able to enchant (https://l2wiki.com/classic/Nebula's_Necklace#Sellers). Will cost you thousands of Fragments since the chance of going 4>5 is about the same as making lvl 5 pendant