Relics - not now, but I almost expect that at some point we'll need them again for our relics... so I'm stocking up on venture coins now - gonna have those ready when the next relic-step requires 50k seals again.
...some people (myself included) are also working on older relics - so we need those seals for them.

I've also bought some stacks of certain materials that I reckon might become useful again - stuff like the achane-webs, for example. HW-crafts re-used those eventually and they went for 20k on my server back then. So I got a stack ready now (storing it on an alt, so space isnt an issue).
There are also mats that I constantly need - like pottash (for music scrolls) or minimum for buff-wheels. And even if its cheap on the market: Its free for GC-seals that I've got anyways.

I'd be all in favour for adding stuff like the mount-speed-buffs to be bought with GC-seals though. Make it require the full 80k or something like that if you have to, so that it will last a while...