How bad do you want the item?
That is a good question. Be sure you don't let the other person know how bad you want the item; otherwise they will have second thoughts about trading it. They will think "If this person wants it this bad, how much will someone else pay?" If you really desire the item, you might want to cave in and pay whatever it takes to get it. But if you really don't need the item, pass up on an overpriced item. "I'll trade you everything I have" generally is not a good thing to say.

20 "bad" items don't make a good item
A common trading mistake is to assume people are willing to trade one good item for a lot of lesser items. This goes against the common selling strategy of getting the most you can for an item. Many people are looking to unload their items for valuable items or items they can use. When space is limited, they are often not looking for more items in return that they will have to unload. You can, however trade a few good items for a better item if the other person likes all the items you're offering.

Trading Reputation
Your trading reputation is very important. If you cheat someone, you can really ruin your reputation in the future if the cheated person spreads the word about you. This can be even easier due to guilds and guild chat. Don't risk creating problems for yourself in the future. Be honest in your dealings and you will be rewarded. If a person somehow messes up and is unable to use the item, don't be afraid to allow them to return the item. This is of course optional, but it will earn you bonus points in the marketplace.

Furthermore, since this is a rare situation it should not affect your bottom line much.

Auction Houses
If you want to keep things simple and don't want to have to haggle or bargain with other players, try the Auction House. It's simple to use! Just search for the item you want and place a bid. Learn more here.