My dad has all these boxes and junk the boxes are practically rotten yet he says he can use them for car parts,and the junk he says can be sold for some money. But the worst part is he won't let me sell the stuff I own and now there is a big pile of boxes in my room that I can't even get rid of. S my question is what can I do to get him to let me sell all my stuff and get rid of the rotten boxes and most of all the junk?

How old are you? Where is your mom? Any older sibs?

Tell him that you don't like having the clutter in your room, that you'd like to have a nice clean room, devoid of boxes, especially boxes of stuff you no longer use. Ask if you can have a garage sale, or donate some of your things to goodwill.

If you are a adult, just do it, no need for permission

I take it you are a teen? Take items out, 2 or 3 at a time, in a school bag/backpack.
Take them to a friend's house, or a relative, who lives nearby.
Ask them if you can have a tag sale around Labor Day.

If the 'rot' is a health hazard, warn dad that you could call the town health dept and they could order him to get rid of it, and fine him too.
If it's a fire hazard, same thing with the fire dept.
If he has a really neurotic obsession, he will just fly off the handle, of course, and it will be tough on everyone to turn him in. Better to squirrel stuff out of the house. Even his stuff, starting with junk that should just be thrown out.
I had to do that with my mother.

If your dad is a hoarder you will need to find a way to move out of the house. He will not change without professional help, and won't get help until he wants to change. It 's a mental illness and logic does not work, nor will threats, or pleading. Or trying to sneak things out. He will just collect more. Do you have other relatives you could live with until your dad gets help?

My best friend is a hoarder and for years I have tried to get through to her. She fought me on getting rid of an empty container from mixed nuts. Her argument was that if she had it on her living room end table, she could put her remote on it and know which of her many remotes worked. I suggested she put the others in a drawer and only have the good one out but she couldn't handle that idea. In other words, a hoarder can justify any piece of crap having critical value. She also kept an old fur toilet seat cover "in case" she needed to re-cover a cat bed. It was on the bed five minutes before the cat pushed it off and has since sat on the hall floor for over a year. If I tried to throw it out, she would invent another intended purpose.

I know he is your dad but he is ill and you need to talk to another trusted adult and see if you can go live in a clean home. You can still see and love your dad but can refuse to return to the home until he gets professional help and is ready to change.