Just wait until the 100cap comes, and you'll watch 9d prices drop through the shitter too. The problem is just that, a huge amount of players cap these days, and +5 is no longer "special." People just want +7. Market gets overflooded with the "old" and yet really nice gear, that hits the market at a new cap, with only a few people actually needing it. +5 3rd tier pieces sell around 10m on Troy, +5 72 weapons for as low as 15~20m, and +7 72 weapons around 50m. I doubt the prices will ever go up either. More and more people are going to cap and get their final sets dumping yet more 8d pimped gear onto the market, and that shit will be around all cap stalling anyways

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The choice with new caps is usually fairly simple to make; either dump your set before the new cap comes, hauling in the big bucks; or keep it so your leveling is easier towards the new cap. [Generally keeping your old set does better as you can sell all of your drops for big bucks and waste your alchemy drops on them as well rather then trying to piece your own set together.]
tl;dr Nice 8d 3rd tier sets are waaaay cheaper to buy then make yourself. Same is gonna happen with 9d once 100 cap hits.