It becomes easier to hide a service, but technically even a service must be reported. Any exchange of goods or services must be reported, and taxes must be paid on it.
Lets say one person babysits while the other cleans house. Who to say they are not just babysitting and cleaning house for free, so will probably get away with it. But if it is on a larger basis, lets say one company provides transportation, and the other provides medical diagnosis... That would trigger the IRS.
There is sometimes an effort by corporations to use barter to hide income. It is often caught.
That is where it gets UGLY!!! So you have to pay taxes on the "fair market value" of the goods or service that you are bartering. In a normal business transaction, the seller wants a higher price, and the buyer wants a lower price. In a barter transaction, because the price (in money) is never paid, both want the lower price to pay the lowest taxes. They will then claim a ridiculously low price... So far so good. Then the IRS will make a ruling that it is the "standard advertised price". The problem is that is often a starting point for a negotiation down to a real price.
So lets say someone advertises a house for sale for $300k. You would have been able to negotiate them down to $270k. Instead, you paint them a painting you could sell for somewhere between $200k and $300k. You would both like to set the price for everything at $100k, to save everyone tax money. The IRS will probably demand the price be $300k, even though there is no way anyone would have gotten $300k for that house.