Version #1 (Rashbam's text): The Reisha discusses a guardhouse that is not plastered (it is sold), and in the Seifa, which teaches what is not sold, it is plastered.

Version #2: R. Chananel's text says oppositely.

Suggestion: [The rocks] are placed on the sheaves.

Rebuttal: There is no reason to require that they are placed on the sheaves, since the sheaves, which are Metaltelim, are not sold!

Explanation (Ri): "They are not placed" means that they were never placed on the sheaves. "They are placed" means that they were already placed, but not [necessarily] that they are placed now.

Explanation (Aruch): The shells were removed, lest they get wormy.

Implied question: Below [the Gemara] will elaborate about this Mishnah, and the cases of a guardhouse and carob tree! (Why didn't the Gemara fully explain these, before explaining clusters of reeds, which is taught later in the Mishnah?)

Answer: This is the Gemara's style. First it explains the [entire] Mishnah [concisely], and afterwards it elaborates.

Explanation #1 (Rashbam): This refers to the opening of the house.

Rebuttal: We do not discuss at all sale of a house!

Explanation #2 (Ri): This refers to the opening of the guardhouse. It refers to the guardhouse of the Reisha, which taught "he sold the guardhouse."

Implied question: If he did not mention date trees, Stam, all of them are sold!

Answer: Now [that he mentioned them] it is worse, for "date trees" connotes two.

Support: This is like we wanted to say below (70a) "except for carob tree Ploni", that he acquires the other carob trees, even though Stam, he does not acquire [them].

Implied question: We should say that he wrote them for embellishment of the document (to avoid any chance of error, like it says below)!

Answer #1: That applies only when he writes a document, but not here, when he [only] spoke.

Answer #2: There, it is written "[mature] date trees, young trees..." Here that he said only date trees, we cannot say so.