One who buys from a non Jew who bought originally from a Jew needs to prove that the non Jew bought it from the Jew – either by providing the document or by claiming he/she saw the original sale (in the event that he/she had also lived on the land for 3 years). But proof that the non Jew lived there for three years without the owner protesting doesn’t help as there is an assumption that one may not protest against a non Jew out of fear. A number of other assumptions about human behavior are made on this daf. One generally doesn’t bring tools and harvest in a field that is not one’s own. One wouldn’t protest land that is unlikely to grow crops or unprotected land whose produce will likely be eaten by the animals, or produce that is forbidden to sell by law (orla, shmita, kelaim). There is a big debate regarding whether or not plowing would be considered an act of chazaka if the owner did not protest.