Here is a short guide, how not to be scammed:
Step 1) Exchange your item for an item of equal value. If you want your +1 Item (Item A) upgraded, get a +1 Item (Item B) in exchange for yours from the person.
Step 2) The dealer puts +2 ITEM A up for trade for +1 ITEM B and the amount of cash he wanted.
This ensures that you always have insurance. If the other person doesnt come through. You lost a +1 item, you gained a +1 item and didn't lose any money.
If the dealer doesn't have any loomed items to trade, this might be a shady guy, don't trust him, you can ask for money in exchange for the item. If the other person has neither an item nor money he is willing to exchange, trade at your own risk.
If you follow this guide, you too can avoid been rightfully ridiculed by your friends for being a complete idiot.