Some simple steps to avoid being scammed/ripped off:
1) Always check the second trade screen, and wait about 3 seconds before accepting--read exactly what will be traded, and gauge the current worth of both items, then accept or reject. If you don't fully know the worth of both items, check a price guide, either here or on one of the many other runescape help sites.
2) Never change your password to something another person tells you, and never tell another person your password. Jagex cannot star out your password in-game (as a lot of people use common words in their password), so anyone saying "Runescape blocks your password, see ********** try it!" is an absolute liar.
3) Never trade-for-modification unless you know the person you are giving it to. This involves trading your item to another person for him to change it (i.e., gem cutting, armor trimming, etc.). However, if they offer the finished product in exchange for your item, by all means accept
(i.e., a cut gem of the type you have for your uncut). Armor trimming is impossible, so don't trust that ever.
4) Items cannot be duplicated by any magic, crafting or smithing trick. If you trade an item for it to be duplicated, don't expect it back.
5) Many people will offer seldom-heard-of items for a very large price (i.e., sliced banana, gnome drinks, burnt bones, etc). Always check your price guides for exact quotes before you pay for them. If the other person can't wait for you to do this, it probably isn't worth it. If the deal was really good, then you haven't really missed out on too much. Remember, it is just a game, and there will always be another one.
6) Some people offer "Really great bargains" on fairly expensive items. Check your price guides, as 50k for a 30k-35k rune scimitar really isn't that great of a bargain.
I hope this helps