The person might just be new to the game and he might not know the standard price, and it would be really helpful to him if you told him the standard price of the item instead of walking away and making cheap snw vis him feel stupid. 

When trading an item, always double and triple check the items in your box and in his. Patience is the key to not getting scammed. It’s also a good idea to let him press ok first.

If you're both telling each other to press ok first, then I suggest pressing it at the same time.

If a trade gets cancelled just when everything is all set (ie, when all the items are in place), and if the person you're trading with says something like "sorry, accident" or "sorry, lag" and he requests a trade again, BEWARE WHAT HE PLACES INTO THE TRADE BOX.

Some scammers will intentionally cancel a trade then when they request again, they put in an item that is of cheaper value.

(Example, you're buying a heliacal +2 +skill, but after the trade is "accidentally" cancelled, and after he requests again, he puts in only a heliacal +1)