A mom at a garage sale silenced her fussy 8yo son by holding her hand over his mouth, and keeping his mouth held shut like that for 10-15 minutes while she was chatting w/neighbors, I didn't know what to say to her or shd I mind my own business?

Normally speaking, an 8 year old should have enough composure to stand there beside his mother without constantly interrupting her.

He was breathing through his nose and not turning blue was a result of the hand over the mouth, so he was fine.

It could be that she had discussed this issue with him and told him that the next time you interrupt me, I'm going to hold your mouth shut, and then she followed through. If he was in physical danger, I would say something, but he wasn't.

Sometimes we can feel an obligation to act in certain situations, but we also have to be aware of exactly how much good we can do vs the harm that we may inadvertently cause because we don't have sufficient information about the action we are observing that upsets us.

Generally speaking, unless there is a clear and present danger or a willful and uncorrected disregard for acceptable human behavior, I tend to pass on getting involved.

i had a comment but a red flag popped up for bad language.  After reviewing my answer several times I found no bad language, no guttural words or anything offensive and it still wouldn't let me post my answer.  If I can't post on this site using good American English containing no offending wording, I'm out of here!

Her business.

Sorry not your business.

He lacks having not been taught respect and manners. She's lacking in teaching him respect and manners. Both displayed poor judgment.