unfortunately what you have gone through is all too common. Estate agents can and will deceive clients – fortunately not all. However depending on where you are 3-6% is about the Norm for Costa Blanca – however certain areas such as Javea and Moraira, have agents that charge much more.
I also know of one UK based estate agent who bring clients over and insist they charge 15% on top of what the agent is charging. I had a friend working for them and he left for this very reason. However he then went to an agent that charges 10% – I guess by comparison it is less than 15% plus the agents commission.
Also it is highly unadvisable NOT to sign an agreement – yet some people still do. An agreement is usually nothing more than an authorisation for the agent to sell your house at the agreed price which will include their commission (stated) including IVA.
Most agents I know locally insist on signatures (we certainly do) then everything is transparent -and available for if and when a buyer asks how much commission is being charged we can produce the sales sheet signed by the owner.
However I would advise anyone looking to buy a property in Spain – never ever give power of attorney to anyone – unless you really cannot make the notary. Then if you have to make sure it is someone YOU can trust – but the advice about independent solicitors springs to mind.
Also be suspicious of anyone who tries to keep you away from the buyers/sellers. There is no need for this if everyone is being honest – so what does that tell you about the poeple who do leep buyer and seller apart?