Mishna Bava Basra 7:4 says:
האומר לחברו חצי שדי אני מוכר לך--משמין ביניהם, ונוטל חצי שדהו. חצייה בדרום אני מוכר לך--משמין ביניהם, ונוטל חצייה בדרום. ומקבל עליו מקום גדר, וחריץ ובן חריץ. וכמה הוא חריץ, שישה טפחים; ובן חריץ, שלושה.
Q1: There's an ambiguity in the mishna, which simply states that "he" has to provide the place for the fence and the two (or 1.5) trenches. Who is "he"? Kehati says it's the buyer, but I'm not sure where he gets that from, because Rashi in the Gemara (107b) says it's the seller. And Rashi himself is stated without proof; the gemara does not make this point clear. So who has to provide the land for the fence?
Q2: The mishna says that one person (whoever that is) has to provide the place for the fence. Does that imply that the fence itself should be a shared cost? If my diyuk is correct, how does that square with the mishnayos in the first perek, where it's pretty consistent that a shared wall is built with joint funds and straddles the border, whereas a wall put up by only one of the neighbors has to be built entirely on his property, and he puts a siman on the outside to show that the materials and the land are his?