Selling a very rare album

Want to get your thoughts. I'm selling my album collection and I have a rare issue from Jethro Tull named "Hand in Glove" that only 500 were pressed according to disclogs and popsike. The album actually has paper glued on the front and back with Ian Andersons face with a slogan on the front and the song names on the back. It is a double album in mint condition. How would you determine or who would you ask how much something like this is worth?