Someone will have to farm the items, so I'm not seeing how anyone choosing to buy them is a problem.
Because this means the game is balanced around the people with the most money doing the farming. Which means the bosses aren't worth doing until they're not a challenge any more.
Alice and Bob both find a boss invite every hour. The boss drops 100 currency worth of loot.
Alice sells the invite to a rich meta player that farts on the boss and collects the loot in 3 minutes. He even gets bonus loot because of MF or modifying the map with more challenges or whatever, lets call it double. In exchange, Alice gets say 40% of that loot's value for the sale, in the form of currency to trade for exactly what she needs. Alice is making 80 currency per hour.
Bob fights the boss. Fighting the boss takes 20 minutes, several deaths that cost say 10 minutes worth of xp, and provides loot that can be traded for 100 currency... eventually. Trading all that loot takes another 10 minutes, and the whole reward is deferred by about a week on average as it takes time to find buyers. Bob is making 66 currency per hour. Supposing they each play 1 hour a day on average, after 1 week, Alice has 560 currency worth of new gear, while Bob has ~230 currency worth of new gear and ~230 currency worth of assorted crap waiting to be traded.
Alice is miserable because farming boss invites is boring and asks for power creep so she can fight the boss. The lvl 100 guy is miserable because farming the bosses is boring and asks for difficulty creep. Bob is miserable because his progression is painfully slow, AND he has to still farm the boss invites AND he has to spend time dealing with dickheads to trade stuff. He moans about it on RPGCodex and plays something else. The SSF guy is getting like 5 currency/hour worth of gear btw, because he can't use anything that drops since it's all hyper specialized.
GGG is happy because the rich traders/boss farmers that play the game 40 hours a week are free advertising for all their MTX every time someone has to deal with them. They add difficulty to the trash mobs that drop the invites and power creep to the top end gear. Now boss invites drop every 2 hours and the boss dies in 2 minutes, so the top end players seem even more prestigious.