From a legal perspective, it is difficult to understand why Eisav's sale of his Bechorah to Yaakov would be valid. One of the main issues is the matter of selling something that does not yet exist- so long as their father Yitzchak was alive, the Bechor status had no tangible meaning. Only after Yitzchak's death would Eisav's Bechorah entitle him to a greater share of inherited property. Until then, all that Eisav had was an expectation, a דבר שלא בא לעולם. The halacha is that אין אדם מקנה דבר שלא בא לעולם, one cannot transfer something that does not yet exist. This rule is specifically applied to the sale of future inheritance in Bava Metzia 16a-b. The Mefarshim give three reasons for this halacha: Reb Elchanan in BB #267 explains that among the Rishonim there are two reasons given, and Reb Chaim proposes a third. According to the Maharam mei'Ruttenberg it is a problem of intent- that since the item does not currently exist, we view the decision of the seller as being legally incomplete. There is no גמירות דעת. According to Reb Yechiel mei'Pariz, the problem is that legal transfer is impossible when the item does not exist- it is not called a מעשה קנין. Reb Chaim says pshat that the problem is that you have to own something in order to sell it. Until it exists, it is not yours to sell.
Similarly, others have suggested that since we pasken (ח"מ ר"ט:ד) that you can piggyback the kinyan- that if one sells a דבר שלא בא לעולם together with something that exists and belongs to the seller right now, both kinyanim are good, here we can say that Eisav sold his current Bechor status regarding Avodah, as the Ohr Hachaim says. Since that currently exists, the דבר שלא בא לעולם part can go along with the דבר שבעולם part.
The Ketzos in that Siman focuses on the result of a Bechor, or any yoresh, waiving his inheritance rights. He says that a Bechor certainly has at all times the ability to make a binding and irreversible statement of waiver, that he declines and abandons his rights to his Bechor privilege. He says there is no need for the Rivash's answer, because there was no kinyan per se here, there was Siluk, withdrawal. We don't need to look for a way to validate the Kinyan here. All that was needed was for Eisav to waive his rights, to do Siluk. Siluk always works.
The problem with this Ketzos is that even if Siluk works, that just makes the Bechor like any other heir. Siluk does not mean that the right is transferable! Eisav giving up his rights does not make Yakov the Bechor. So what good is the the Ketzos' teretz of Siluk?
I heard an excellent teretz in the name of Harav Shimon Moshe Diskin, Rosh Yeshiva of Kol Torah. To explain this puzzling Ketzos, he uses an even more puzzling Gaon, and by putting them together, both become clear.
Despite my dislike of using Latin when an equivalent phrase is available in Hebrew or English or Yiddish, I have to use this phrase that exists only in Latin: Ignotum per ignotius - offering an explanation that is harder to understand than the thing it is meant to explain- explaining the incomprehensible with the doubly incomprehensible. He answers the question about selling Bechora using a Gaon that is even harder to understand.
The Gaon in Ki Seitzei seems to say that Bechora depends on conception. The Gaon apparently holds that the child that is conceived first is the Bechor, not the child that is born first. This is contradicted by innumerable Chazals, including a Medrash quoted by Rashi in our parsha (25:26).
A great deal of effort has been put into explaining what the Gaon meant. Indeed, the Netziv says that anyone that says that the Gaon said this is oiver the issur of Lashon Hara, because he never would have said such a thing. On the other hand, it has been brought down by reliable sources, including Reb Shlomo Kluger (in CM 278:1) from Reb Chaim Volozhiner. The Noda BeYehuda in his דורש לציון, Drush 4, also suggests, at one point, the exact same pshat in the pessukim in Ki Seitzei, so shver as it is, if we don't understand it, the problem lies with us.
Harav Diskin says that the Gaon didn't mean that Bechora is determined by the moment of conception. What he meant is that the moment of conception is a factor in determining the Bechor. However, the moment of birth is a more powerful determinant, so when there is conflict between the two, birth order is dominant. Where, however, the first-born waives his claim, the moment-of-conception claim is no longer inhibited by the superior claim of the moment-of-birth, and so it becomes the legal determinant. This is what the Ketzos means. Waiver certainly would not be enough if Yaakov was just a regular heir. But here, Yaakov had a claim, albeit an inferior claim, to the bechora, based on earlier conception. Once Eisav's claim was waived, Yaakov's hitherto potential status of Bechor was actuated.