If you're buying on a small scale then it's unlikely you'll be banned. It also depends on weather or not the person you're buying from is being monitored by jagex.
Buy from a smaller seller that deals with less gold leads to a lower ban rate. Also if its a small amount and on 07 it's pretty unlikely I've sold/bought quite a bit without trouble.
They rarely ban low scale buyers as they know that if they ban them they might not renew a membership whereas a gold shop will likely continue their business and renew a membership with their game.
Jagex really doesn't look for those who buy, they spend much more time looking for the big game sellers. I wouldn't worry too much about it but you can reduce your chances by staking it or adding other items to the trade.