Avira
selling chometz is not something new, its brought down in the shulchan aruch siman 448
the source is a tosefta and therefore chazal
and the biur hagra there does not write anything against selling it
the mishna brurah does not write against selling chometz
and therefore it is a chumra and its a chumra against chazal
if a person wants to be machmir not to sell thats one thing but after it was sold the halacha is its allowed (supposing the sale was done correctly)
please see the aruch hashulchan y”d siman 116 were he says that someone who is machmir in a place that the poiskim were meikel could be considered minus (theres also a maharshal brought down in hilchos busur vichuluv that says the same)
this is all except for the issur of baal tashchis for throwing out food that is halachically permitted
– at least donate it to the less fortunate