I am going to say something right now; this guide is not to teach you how to scam, this is to teach you how not to get scammed. I was planning to post my huge list of scams here, but then i tried posting something similar on the forums on Runescape and got it locked, after i realized what I had done and requested it. I realized that while trying to stop scamming i would be creating more of it. Even for the non-scammers, if they were to read a whole big list they might say "Hey, if I have all of these, why not scam?" So I deleted my list. However, I posted some tips that wont tell exactly how to scam, just some minor details.
Some scam warnings;
If there is a low level player saying that they will trim armor or sell an account, they are probably scamming. People do this because they make a new account so that if they scam and get banned, then they don't lose any of their good accounts
Never agree to terms that you don't agree with just to get something because you are desperate, it could cost you a lot of money or it could end up very badly
If something happens (like you don't have enough room for a trade) and they say for you to give your part first, decline, because they are most likely scamming you. Make sure you know the person if you do trade though
Never drop items to trade; always use the trade system, because there is no guarantee that they will do their part of the deal just because you do
Be very wary, and don't trust any old stranger. As Jagex states in their anti-scam guide, "If it sounds too good to be true, then it probably is." I used to trust almost everyone. But now...after all of that i barely even trust friends that have been really nice to me. If someone says something is free, that's the time to be most careful.
Your password is for you ONLY! Jagex NEVER asks for anything about you, including your pass, and anyone who says they can get you something etc. if you give them your password, THEY ARE LYING! I would even avoid telling it to your real-life friends in Runescape unless you really trust them. A tip just in case you do get password scammed and someone changes your password to prevent you from getting your account back, make sure you set recovery questions!
One last statement. Although it is not a good idea to trust everyone, don't assume that its necessarily a scam. For instance, if someone says they are cutting gems for free, they might be after crafting xp, and not your precious gems. To make sure of this, ask to trade your uncut gems for a cut gem. This way they still get xp, but its better for you. (yes I know I went against my code and let out a scam)
If you do get scammed, report them immediately. Jagex only looks at the last 60 seconds of trade/chat after someone is reported, so you can get it counted against you instead of the scammer if you report too late. A lot of scammers log out right after a scam. This is completely rude, but unfortunately, you can't do anything about it. Jagex ignores a report if someone isn't even on at the time, and reporting them after they get on later doesn't work either.