I have sold my property through them. My property was sold prior to auction so I do not have any experience of the auction.
I found them to be trasperant. I however asked the right questions and was firm in my dealings through out and had no problems.
As a bidder I should look for the following:
1) Title of the property match it with passport/identity card to know the seller is the owner.
2) Any debts on the property mortgage etc.
3) Last statement of Council Tax IBI
4) Receipt/proof via Bank statement of service charges for the block
5) speak to neighbours and take on their input . In Spain people are open and will assist you if you tell them that you are thinking of buying.
Normal prudance where ever you buy. In UK this done by the solicitors.
Where were you planning to buy, Maybe I know the block ???