I can understand your concern camposol but I shouldn't worry too much, you are after all using a Gestor, you do have recourse with them if anything goes wrong.
You should leave the office with the money and a signed and stamped compraventa (Sales Contract) this gives you a fair bit of security as you will have the buyers name, address and NIE number.
In our area (Malaga) it can take as long as three weeks for traffico to process the change.
Keep photo copies of all the car documents just in case.
the carfax site would be no use to me as the seller
Of course it would, What mariedav is saying, is that for added piece of mind, after say six weeks go to a site like this and buy a report on the car which by that time should show your buyers details as the owner.