Ok i hope this will help you prevent any kind of scam.
1. Sell item for GP/GP for item
Do the trade on the same trade screen (of course you all already know this)
2.Sell item for Paypal/Paypal for item
In this situation obviously someone have to go first on order to do trade and scam are really common here PRECAUTION is highly needed here.First of all if your trading a little amount worth of item to someone and HE IS TRUSTRED you rarely get scammed but if your trading a huge a huge amount TRUSTED OR NOT you should a OMM.But remember do not trade with random person you must chose your trader correctly or otherwise the chance of getting scammed are high.
3.Runescape bots for item/gp/Paypal
First of all here you MUST and i repeat MUST demand a receipt of the auth being bought to make sure it's not a dumped one.Btw that same situation happen here,You must chose once again a trusted person if you don't feel like paying a OMM or maybe find one free but once again TAKE CAUTION
4.Account for gp&Paypal
If your buying a account with a registered email i 100% recommend using a OMM due the fact that's complicated and the chance of getting scammed are high once again.Also get some proof that he's actually the account owner.(If you don't feel like using Teamviewer use Skype screenshare so you can see everything).
5.Little Paypal trading advice
If your buying something with Paypal money and your trader want you to type a message with the payment you must be careful because some kind of message make it that you can't get charge back if you get scammed.(Once again only trade with trusted person or USE A OMM)
After you have read and you probably did because your now reading this text xD you will understand the importance of using a OMM and taking a lot of caution when trading.
I'm not trying to say that normal person are not really trusted or something but i have seen many ''trusted'' person scamming when it come to high amount of trade.