I bet Godslayer will be much, much earlier available to buy, if it will be a popular demand for gold/rmt. The thing is I bet people do not buy often even Gryphon hearts, and the amount of skilled players interested in selling this kind of thing will remain very low. If WOW endgame is 99% raids (as what I heard sorry if Iam wrong), there is A LOT of offers for end-game achievements. In ESO there is a TON of players that don't even know trials exist (due to a vast amount of other content and different root of game series overall) and they are more happy than skilled raiders.

tl;dr all achievs will be sold much earlier than year(s) if demand will be high (but imo it wont)