Instead of naming and shaming bad agents why not set up a good agents charter. Have a website where prospective purchasers can clearly see the names and contact details of reputable and client focused agents. I think a maximum of 5% commision is more than enough. Even if you worked on a split commision basis with a uk agent you would still receive 2.5%. On a 200,000 Euro property that still works out at 5000 euros.
The other way to approach it would be to offer a tiered commision structure this would still enable agents to make a decent commision on cheaper properties.
I think purchasers also need to sharpen themselves up and do there homework, you wouldn’t think of buying/selling a property in the UK without finding out what the estate agents commision would be so why do so many do it in Spain!
Ronnie