I am an habitual collector. Over the years I've collected:
Stamps (inherited most from my grandfather)
Coins
Baseball cards
Old Macs
Books (I read them all, don't just collect)
Military rifles
...and some other stuff. Of all that, I've never gotten rid of anything unless it was for the purpose of acquiring something else similar. I might get bored of a particular collection, but in that case I put it into storage rather than selling it.
With a house that was 900 square feet, to a slightly smaller apartment, back to a house that size, I had to drop the habit of hoarding or flipping.
Stamps - I gave them all, a couple of thousand dollars worth in the 1980s, to my cousin. They were stored away in the damp town I live in and they got moldy, messy, and sticky.
Coins - I did get rid of most of those, but to buy guitars so it was a transferred addiction.
Plastic models - One day I gave those model airplanes, cars, and ships to a young model builder who later took to mechanical things and went to a good engineering school.
Guitars - Met some rock stars, producers, and some rising stars, who all pretty consistently told me to pick a guitar or two and focus on improving my playing and songwriting, as one only has only so much time and attention in this short life. More than once people in the music business told me that in most cases, either you are a music collector (records/CDs, instruments, etc) or a starving musician who pays their dues and hopefully collects royalties/hit records/contracts. This isn't the case 100% percent of the time, but still 99% percent of the time.
Girlfriends - Definitely the dumbest thing to collect. It's OK, at least in this culture, to have one. But I don't live in a country, and you probably don't either where you can have them like a huge bag of potato chips. My generation didn't have AIDS to scare us into a more sensible approach and many saw abortion as a form of birth control. Anyway, collecting girlfriends is a great way to get hated.
Computers - (and extra perepherals) I soon realized that I gravitate towards one machine, regardless of how many others are sitting in front of me so I gave most of them away.
Books - Now there is the computer/internet for reading, and I have actually found a use in the library, which has a collection far bigger than I could ever hope to have. I gave away most of my books to people so they could learn how to read, and gain knowledge at the same time. I used to keep all of my school papers but these days anymore, I just hang onto some law school stuff and a few key MBA books. I have a Bible and just a small stash of books on history, and the three Rs.