SETS, WEAPONS AND THE ELITE PVP LOOKS
How it is played
Faction
Armour type
Loot
Server
Transmog unlocks account wide, so a set farmed on one character dresses every character you will ever make, including the ones that do not exist yet. That is what makes it worth paying for once: it is the only thing on this page that does not reset with the expansion.
Legacy raid and dungeon sets can, and we do. Elite PvP sets can only be bought in the season they belong to, so that row is a this-season job with a deadline on it. Anything removed from the game is gone and nobody can sell it to you.