Do not trade for anything that cannot be added into the trade window within Steam. The most common examples of these scenarios are the following:
Trading items/gifts for money outside of the Steam Community market. You cannot add wallet credit or any form of money into the trade window. A common example of this is trading for items/gifts in return for money via PayPal.
Trading items/gifts for CD Keys. You cannot add a CD Key into the trade window. CD Keys that are offered can be for a different game, fake, used or region restricted. A common example of this trade is trading items/gifts in return for a promotional CD Key that is never given to you.
Trading items/gifts for nothing in return in the first trade and expecting to get an item or gift in a later trade. There is no reason to not trade everything in one trade. You may add unlimited items/gifts to a single trade. A common example of this is adding your items/gifts in one trade. Then the other user states that they will trade you their share or the rest of the items/gifts in another trade, but never do.