Question

Hello Rabbi,
In the Mishnah in Bava Metzia 49 it says: “Rabbi Tarfon ruled in Lod regarding overcharging…. and the merchants of Lod rejoiced. He said to them: one may retract the entire day. They said to him: let Rabbi Tarfon leave us as we were.”
My question is: why couldn’t the merchants of Lod rule like Rabbi Tarfon only with respect to overcharging, while regarding retraction follow the view of the Sages? If there is one Jewish law in which they follow Rabbi Tarfon’s opinion, does that mean they must follow him in everything else as well?
After all, the Talmud says (Rosh Hashanah 14a, and elsewhere) that one may follow the leniencies of Beit Shammai and the leniencies of Beit Hillel, and only if it is contradictory within the same law (such as impurity in vertebrae, etc.) is it forbidden to follow the leniencies of both this one and that one.
Seemingly, in our case there is no contradiction if one follows Rabbi Tarfon on one point and the Sages on the second.
And to broaden the question: is there such a thing as “appoint for yourself a rabbi,” meaning that you are obligated to follow one rabbi in everything?
Seemingly, the Talmud is full of cases proving otherwise (for example: Berakhot 42b, “Rav said it exempts… all of Rav’s students said it does not exempt”), while on the other hand sometimes there does seem to be an implication in the Talmud that there is an obligation to follow a particular rabbi (see the Talmud in Shabbat 37b: “…you, who are close to Rav and Shmuel, act in accordance with Rav and Shmuel; we will act in accordance with Rabbi Yohanan…” and this requires examination—what difference does it make that they were close to Rav and Shmuel…)
Thank you in advance