you won't get banned unless you're already being watched other then that chances of you getting ban is very low
Verrrrrrry low. I've bought and sold gold for years with no issues. I usually buy gold on a lower level alt and transfer it to my main though.
Never been banned and always bought/sold GP. I think the only way your getting banned is if your a streamer/in the spot light of other RS players.
The biggest chance of ban is if you are using a botting client and botting while buying it. Otherwise there is a minuscule chance.