Quetz, you are right, it is not safe for strangers to come into your Mom's house that way. How is your Mom advertising the sales? Is there a way to cut off her advertisement? I am assuming it's is through the newpaper classified ads. Could you try calling the paper's classified section and making up a story why they couldn't run any more ads? Perhaps help her organize a yard sale once a month or go to a flea market one a month to get rid of the things she doesn't want. Even have someone else sell it from a consignment shop. They truly don't see the danger.
I had to giggle a bit about your son spelling with spaghetti. You have to applaude the creativity of the new teacher. I do hope your son has a good year
You are doing well. It's tough to set limits and live within them and even harder to let all those barbs pass you by. But that's what you have to do.
Hope you find an answer to your problem.