Concerning the issue of not acquiring an item through sight alone, it appears that the gemara wanted to use this mishnah as the source for all other mishnayos that one cannot acquire anything (see Tosfos on Hefker) through sight alone. The gemara concludes, however, that the additional words "kulah sheli" refers not to a lost object but rather to a purchase. Therefore, according to the gemara's conclusion, how do we know that one cannot acquire a lost object through sight alone?
Aharon Levine, Baltimore, MD
Your question is based on the implication of Rashi (DH b'Alma), who writes that "there is no Tana who teaches us that one can be Koneh it in a way other than Hagbahah" (which is not like the Tosfos ha'Rosh). Your question is a very good question (indeed, a number of Acharonim ask this question, as cited in the Otzar Mefarshei ha'Talmud).
Indeed, the words of Rashi seem perplexing. Why do we need a Tana to teach that one is not Koneh with Re'iyah? From where would we think that Re'iyah is Koneh? The verse itself implies that the object must come into the person's hand in order for him to be Koneh it, as is clear from our Sugya! The Shitah Mekubetzes asks this question on Rashi in the name of "Mori ha'Rav," and answers that Rashi is saying that even though there are logical grounds, and the inference of the verse, to teach that Re'iyah is not Koneh, nevertheless since the Mishnah does not say so explicitly, if the Tana now teaches this law in the Mishnah it is not considered an unnecessary, extra teaching.
Accordingly, we can understand as follows. Rashi's intention is not to say that the Tana must teach (through his choice of words in the Mishnah) that one is not Koneh with Re'iyah, but rather that if the Tana wants to teach it, it is not considered an unnecessary teaching to do so. According to the Maskana, the Tana did not find it appropriate to allude to this teaching at all. (We find something similar to this with regard to Shevu'as Ed Echad, and Birkas Mezonos, which are not written explicitly in any verse or Mishnah.)
Similarly, the Mishnah does not discuss the Berachah made on "Besomim", even though it is a whole category of its own -- "Birchas ha'Re'ach" -- that is not discussed explicitly anywhere in the Mishnah (except for mention of the fact that there is a Berachah on Mugmar, in Berachos 7 Perek).
Kol Tuv,
Yitzchok Zirkind