Just assuming good games cause people to spent more then regular monthly fee, is it good if people will buy gold and buy for example soul gems to play more in PvP?

Should it be forbidden/punished or just tolerated?

Unless the Game developers themselves are selling it, it will be forbidden.

Unless you can purchase in game currency for real money in the cash shop, then it will most probably be forbidden like a lot of MMOs, anyone caught buying or selling can be permanently banned, and rightly so.

Yep, this has been true of all properly policed MMOS. It dirupts the ingame economy, gives people an unfair advantage and just as often the gold buyer finds that they've had their account hyjacked because they used a seller who was just after account details.

I rememer SWG and several dozen accounts got banned in the space of a day or so for buying and selling gold.