In short, the answer can be Yes and No. Please be patient with me.
It's true, as you could risk being arrested by Jagex the game's developer. Many people believe that runescape old school buy gold is a bad idea for the economy by controlling the flows of cashI'm not able speak regarding this at the moment.
Yes, but, as some of the responses here, it's not hurting anyone. If you think about it similar to the selling of accounts just increases the excitement for the game. Assets that are not moving are likely to be unused for a long time when transferred to another individual and then it will find a new life which creates another cycle for that account or for the gold.
My final conclusion? It's somewhere between bad and good.
I have answered this question in a separate one that might be of help:
It's a great site to determine the quality of buy rs3 account selling websites are safe. The reviews are written by users, making them appear impartial and free of bias, but take from it what you want I found the link on one of the most trusted forum.
Best of luck!