1) THE RULES OF HOW TO RULE
QUESTIONS: The Gemara searches for the source for Rav Mesharsheya's opinion that there is no such rule of "[wherever] Rebbi Yehudah and Rebbi Shimon [argue], the Halachah follows Rebbi Yehudah." The Gemara cites several incidents in which the Halachah was decided in favor of the opinion of Rebbi Shimon, and it refutes each source by saying that the particular incident is an exception to the rule.
(a) Once the Gemara gives that answer to the first attempted proof, why does the Gemara continue to ask from other sources and to give the same answer? (RITVA)
(b) In addition, the first three cases that the Gemara cites involve questions concerning the laws of Eruvin. In all three cases, Rebbi Shimon's opinion is the more lenient one. Perhaps the reason the Halachah follows Rebbi Shimon in those cases is because of the rule that in the laws of Eruvin, the Halachah follows the lenient opinion! (MAHARSHA)
(a) The RITVA explains that the when the Gemara continues to ask from other cases, it knows that those cases might also be exceptions. However, the Gemara assumes that if we find three places where an exception is made and the Halachah follows Rebbi Shimon, then perhaps that creates a Chazakah that negates the rule that the Halachah does not follow the opinion of Rebbi Shimon.
(b) The Ritva (see also RAMBAN) answers the second question as follows. If the reason the Halachah follows Rebbi Shimon is because his opinion is the lenient one, then Rav should have stated simply that "the Halachah follows the lenient opinion," and we would have known that the Halachah follows Rebbi Shimon in those cases. The fact that he specifically said that "the Halachah is like Rebbi Shimon" shows that he was not ruling like him merely because his opinion is the lenient one. Rather, Rav's wording implies that he did not accept the rule that the Halachah follows the lenient opinion in the laws of Eruvin, and he nevertheless ruled like Rebbi Shimon.
(The Ritva also answers that the Gemara indeed could have answered that the reason why the Halachah is like Rebbi Shimon in those cases is because his opinion is the lenient one.)
2) BUYING ITEMS FROM NOCHRIM DURING THEIR FAIR
OPINIONS: The Beraisa teaches that one may buy animals, servants, houses, fields, and vineyards from Nochrim during their fair, even though doing so might cause the Nochrim to go give praise to their idols for the sale. Why did the Rabanan permit such purchases?
(a) RASHI (DH li'Yerid) explains that when one buys land or houses from Nochrim, he diminishes their power. Moreover, if the property is in Eretz Yisrael, then he fulfills the Mitzvah of settling the land.
When one buys mobile property, such as animals and servants, he elevates them by granting them the Kedushah of being owned by a Jew (RASHI, DH Behemah).
(b) The RITVA writes that one is permitted to buy objects from Nochrim during their fair because it is considered a "Davar ha'Aved." If one waits until after the fair is over, he will not have the opportunity to buy those items.