It does devalue the market for max level characters, but that was a black market anyway. In theory, bought characters (if discovered) could get the account banned.
However, I think it's been a long time since you had people power-leveling characters to max to flip them for cash was common. My understanding was that in more recent years, it's been far more common for people to try to steal ("hack") the accounts, and then try to sell those compromised accounts.
When accounts were (or are) not compromised, using cheats and exploits was also common, which again could put you at risk of the "banhammer" if your account comes on the radar of Blizzard.
It was still a short-cut (with risks) for exchanging money for time investment, unofficially.
But again, the lack of a good solution to the original problem that new players are effectively cut off from the content of veteran players is what created an incentive for this type of criminal behavior.
That's one of the big problems with MMRPGs. They eventually become so old that only those heavily invested in it can find it fun and compelling to continue playing. The gap between newbie players and veteran players will continually expand.
However, I still maintain that there are better ways of bridging the gap between old and new players, rather than requiring huge time investments or more money.
Or... maybe MMRPGs are just a flawed idea in general.