All our family hoards to some extent - Aunt Nell was just a bit worse than the rest of us. My mom and dad were in the same house for almost 50 years. We got it cleaned out when they moved! The thing to do is to designate a spot for "trash", put a big tarp on the ground, and once a week have a guy with a truck come and haul it all away. Alternatively, you could rent a dumpster.
The thing is, there WAS a lot of nice stuff in there, some of which went to their new house and some of which found its way into the clutter in OUR house!
Hopefully we will sell dad's '62 Studebaker soon - we have three interested parties although nobody's brought us a check yet. THEN we can get the garage organized and have the Mother of All Garage Sales.
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Or put it out at the curb with a sign that says "free". It's amazing what people will cart off.