Where the land is held under a title deed and even if you are shown the original title deed there is need for verification with the Deeds Registry on a number of issues. A deeds search (preferably done on your behalf by a lawyer) will detect mortgage bonds, caveats and other endorsements, if any. You also need to physically see the land being sold and the identity of the seller
Where the rights in the land are to be transferred to you through cession, you need to have sight of the agreement which gives the seller rights in the land. You also need to check with the person or institution which will process the cession for example the council. In some cases there may be conditions for cession to go through.