Buyers are asking for "numero de folo, finca y libro''

The buyer needs that information in order to request a title report from the National Property Registry. I've always asked for a copy of the registered deed so I could request such a report and compare the registered owner's name against the seller. If you are buying beach property or property along a navigable river, you need to see the OCRET registration and/or the coordinates in GMT so you can request something similar from OCRET.

Beware of people selling property, especially beach property, with "derecho de posesion". a seller can even have a paid tax bill in their name but still not have legal ownership of the land, the muni doesn't check ownership if someone comes in and asks to pay some taxes, they just take the money and issue a receipt. I ran into this a couple of years ago. I avoided the scam by asking around the neighborhood when I wasn't with the seller.