You radically, and I mean radically, decrease the likelihood of a deal when you are looking for a trade. The odds of you having what the other guy wants and him having exactly what you want are pretty thin. That's not to say impossible, but the force is typically not with you.
While there are audio enthusiasts who would be happy to trade for a wide variety of things just to say "oh yeah, I had one of those for a while..." most of us get very specific ideas about the item we're looking for when we're changing or upgrading.
You also have the problem of how to handle disparities in value. The other fellow has what you want and you have what he wants, but he wants an even trade and you want $500 on top of trade. In a normal cash sale each of you just move on to the next seller who has what you want. In a trade, trying to get a new deal with someone else has just compounded already poor odds.
Using money as the vehicle in the selling process eliminates all of those issues. Sellers get a vastly wider audience and buyers find it easier to get exactly what they want.
In short, this is all basic economics. It is why money was invented in the first place.
