Avira
“The issue the Gaon had was with selling it while it remains in the jews house/store.”
I’m not sure how you know that. As Far as I know the Gra did not write anywhere opposing the sale, including in his comments to Shulchan Aruch (448:3) where sale is mentioned.
Masseh Rav does mention that he avoided buying from Chometz that had been sold, thoughg it does not say why.
Do you have a source indicating that he was “very, very against it.”?
(To be clear I’m not saying he wasnt agaisnt it, (though “very very” seems like a stretch) I am saying that just becasue he was agasint it does not mean this is something that people should follow today for all the reasons I shared from Rabbi Reisman in previous post)