This account selling started way back in the first Ragnar ok era. I haven't done it my nephew who is a Ragnarok player always do this. He creates an account, build it up and sell it.
Question is, why other people buying already built up accounts? It's because they don't want to go through the process of being a noon, or they don't have as much time to build their own characters from zero. Understandably, there are lots of cyber bullies specially on online games. These bullies (who probably bought an account as well) love to pick up on newbies.
The higher the level of a character plus if equipped with rare or unique gears means it's more expensive.
Another way to make money on online games is selling rare items.
Some also earn money for building up the character of other players.