you can use Dust to unlock Legend & Eternal items in the Codex, it just costs a lot. after unlocking an item, it costs less Dust to buy it, but it is one item per purchase, so if you want 5 Eternal Items for a chance to get an item with good affix values or a Crystal affix, and it is 500 Dust, you will spend 2500 Dust to get all 5.
there have been some players who have said they have made Eternal Item only Builds, but they aren’t that easy to make. as you can see from the Link I shared, only a few Crystals can be used to Craft Eternal Items. this will force you to find Eternal Items that work well together and that have affixes that you can use most, if not all, of the affixes on the Build you are going for.
Jasper is one of the Crystals that can be used on Eternal Items, so you can use items from other Classes if they fit into the Build you are trying to make.
just remember, Eternal Items double the values of affixes, so if you needed 4 Legend items for a certain Build, you would only need 2 or 3 of them if they are Eternal, depending of the affixes you are looking for.