We hear from today’s rabbis that it’s better to do the Mechira this way, and better that way (same time zone, Kinyan Sudar, more kinyanim by the agents, an observant Jew, Hefsed, renting the rooms where the Chametz is kept, additional boilerplate, filling out detailed forms, etc. etc.), but no matter what, it’s always completely valid (please stop smirking!).
But here’s a Tosafos everyone learned in Yeshiva, B.B. 76a:
ספינה נקנית במסירה. בסמוך מוקי לה ברה”ר ופירש רבינו שמואל דמסירה קונה ברה”ר משום דלא אפשר התם במשיכה אבל בסימטא דשייכא משיכה לא קניא מסירה שהוא קנין גרוע וקשה לר”ת וכי משיכה ומסירה מצות הן שאין לעשות אלא מן המובחר ואור”י דאין זו קשיא…
Kashya or not, the bottom line sevara in bold remains true. (Like the rule: No such thing [generally] as Lechat’chila in De’oraisa.)
I asked many rabbis this question and never got an answer (except quibbling about examples). The smart ones at least have the decency to look like someone opened the bathroom door on them by accident.
The real reason for all the theater is the realization that the whole thing is a joke בלבו ובלב כל אדם, a travesty, an embarrassment, lipstick on a pig.*
*I refer to the Mechira mentioned by the Beis Yosef. Of course, since then, the procedure has been rendered progressively worse and worse (even if the original ceremony had been valid), see a list of some changes on p. 52 here.