The boss in sprite world is not only relativity easy, but drops Dex pots as well. Not to mention the mobs in it make it the easeiest (excluding pirate and spider) to rush through. And as buckethead said, you can also get other things from the boss and mobs, sprite wands and incs to just name two.
As for the selling of dex for def, its a 3:1 ratio (Dex : Def) then for def to vit is a 2:1 ratio (Def : Vit) Vit to life varries baced on the seller of the life, but usually you can get one for 6-7 vit.
If you wanted to get it cheaper you can just buy def. Most life traders are accepting (As of the 8th) 8( to 12) Def for a life.
Trading life potions for items should not be as expensive as he (XX22) said. Feel free to use the guide below for more percice prices on everything worth buying and selling. (Pots included)
Lots of love