As the end of Warlords of Draenor expansion is nearing a good deal of guilds are selling Hellfire Citadel boosts in various difficulties.
There are many kinds of boosts that WoW guilds are putting on sale for HFC, and they seem to be very popular. These boosts enable casual players who have gold to spare, to quickly get through the current endgame content and gear up to perhaps leave the casual approach.
Boosts that are currently running include full Heroic HFC runs, Archimonde kills in Heroic difficulty, full Normal HFC runs and even some Mythic kills are now being sold. The prices are quite heavy and on my server, for example, a full Heroic HFC boost costs around 200,000 gold. Archimonde Heroic kill goes for roughly 25,000-30,000 gold and the top-notch guilds are selling Mythic kills in Upper Spire for roughly 80,000 per boss.
I am also quite convinced that players can buy PvE boosts for real money through many sites that offer such services but many of those require one to share account details which is never a good idea.
It seems slightly strange that so many people are buying PvE boosts when there are only a few months left till Legion kicks off, although many players probably want to get all the obtainable PvE achievements before the next expansion. But gear-wise, I am pretty sure that the items you receive from quests in Legion will be good enough to level up to the new cap unless you really want them as transmogs.
If you lack the gold to buy boosts, it is still not too late to catch up with the current content through traditional ways, and each server most likely has many guilds that still do normal HFC runs that enable a player to progress further to Heroic and then to Mythic raid difficulties. What you spend your gold on, is entirely up to you, just remember that one boosted boss kill in Mythic difficulty costs roughly the same as a WoW token that gives your 1 month of game time.