Hi there, I need to admit I have never get scammed yet in this game (besides buy somethings more expensiver then they real should worth), the trick t do not get scammed is very simple.
In moment you go start a trade session you make your offer, other player make their offer, Guild Wars has already a anti-scam option the "Accept" button, so all you need so is: wait for other player make his offer, and if you see EVERYTHING is correct (make sure is 60 Plat and not 60g for exemple) ONLY then you press Accept
Now case he press Modify and change it?, no problem Guild Wars automaticly cancel your "Accept" button so you will need to Accept again

, so before you Accept you ALWAYS verify everything is correct, it might take you extra 5-10 seconds to verify you offering the right items and receiving the right stuff also, but this way you never get scammed.
I mean in TRADE scams, in Run or other Services types exist bigger risk of scam but always exist 1 solution for prevent them.
Just remember the: ALWAYS verify everything before press Accept, I know, when you selling some item and some one offer you an huge offer we have tendence to press Accept right way before he change his mind, but this cases most times are scams instead he offer 100k will offer 100g, so just make full sure everything is correct before press Accept, and you never get this scam types
And extra attention in ITEMS Scams, the biggest Scams I saw so far was a seller selling an item, but instead he show 2 items you say "I just want that one" so he press modify REMOVE both and add 1 crappy item with same design as the one you wanted so you don't notice and accept the scaming deal
For exemple: "WTS: Malinon Shield and Grognar Sword" you talk to him when go start trade session he show you both (but you jut wanted the shield) so he say "ahh ok then" he remove both and add a crap Shield of the Wing (same skin as Malinon) and as you see is the SAME skin and you press Accept, well congratulations you got scamemd and probably he added you to his ignore list.
I just hope, this small reply might help some people to not get scamemd so often anymore, I know that prevent scamming is nearly impossible, but if you take always close attention to what you buying/sell before press the Accept you can prevent lot of scams
Bye