Never heard of them, but followed your link to look at the website. I can't immediately see anything about listing your property for sale with them, let alone the fees you mentioned. Can you post a link to that specific page?
It seems to me that they are acting as a buyer's agent - "WE DON'T SELL SPANISH PROPERTY, WE HELP YOU BUY IT!", which implies to me that they will try to source a property for a buyer, then either claim a portion of the selling agent's fee, or charge the buyer a fee directly - but it's not clear to me at all from their website what they really do. Perhaps it is just a property for sale listing?
Either way, personally I would think you would be better off listing your property with some local agents, who at most will ask you to pay for a nota simple from the land registry (about €15) up front. OK, they will want 3-5% commission if they sell it, but you can allow for that in the advertised sale price, and there's probably a better chance that they will actually find a buyer. With a UK address, and only a Spanish mobile number on their site, I can't help feeling that you won't get very much for your 500-700€. But that's just my hunch......