Avira
Because this isnt about not relying on the sale
This is not allowing others to rely on it.
Rabbi Reisman is very very very* opposed to this “Chumra”
buying from a yid is a mitzva deoraysa.
chamatz shevar al Hapesach is a knas derabanan. It is absurd (his words) to suggest that Chazal would issue a knas agasint this fellow who follows all Gedolim who allow mechira. He goes to his Rav sells his chometz and surprise chazal issue a knas???
Aye the Gra?
Ok so if you are very medakdek on following the Gra, maybe it makes sense. but if not to adopt this chumra is “absurd”
As to why the Gra held it. Who knows
1) not all things in Maaseh Rav are reliable.
2) Maybe he thought the mechira wasnt done well in his locale
3) Practicly it wasnt the same people didnt have storehouses with product as much as today.
4) furthermore might be better off buying from yid today. You know he sold his product, it is deffinitly NOT chometz sheavar al Hapesach when buying from goy you don’t know who he bought from over yom Tov could be a Jewish distributor
*I use all those verys because I heard it from him many times and he used strong language. Do you have a source that the Gra was “very very against it” Maaseh Rav says he didnt buy