The few times I sold diamonds, if it was to a low level, I wouldn't send any items before receiving the gold. I just didn't trust people enough to risk it, since there are a lot of scammers in RoM. I knew a lot of people that did the same, so it's not unexpected.
If you're buying diamonds, don't buy from anyone below level 60, at the very least. And even then, take caution. Generally, high level players selling diamonds in world chat are trustworthy (and if they're not, it's common to see others warning against buying their diamonds). Especially if you see them as a frequent diamond seller. But don't be surprised if they expect the money first, depending on your level.