Estate agent's fees (unavoidable for some, who for whatever reason can't sell privately), whilst pretty outrageous in Spain, are still far less than the other costs mojito is referring to: plus valia on the sale, and then transfer tax (plus notary & registry fees) on the purchase. What Kavanagh seems to be proposing is underdeclaring - a risky strategy these days, not to mention potentially leading to greater CGT in the future.
I don't see how a property swap scenario would avoid any of the taxes involved in legally transferring ownership. As mariedav says, it's a big stumbling block for many who would like to move, and I believe one of the root causes for the stagnation of the property market since the crash 10 years ago. Perhaps the government doesn't want to offer any incentives, for fear of the market overheating again. It does make me wonder how any new entrants to the market still see Spanish property as an "investment", though. The costs of buying and selling are surely likely to exceed any potential financial gains to be made?
Ahh, for the heady days of the boom....!