Actually I do keep track of most of my inventory. I guess I'm weird in that way
In listing what I do below, it sounds like I am strange, but I'm really not. I just like to be organized.
For my fabric, I take a picture or if buying online, make a copy of the picture of the fabric and keep it all in one word file, just pasting the picture in word and writing under it the name of the fabric, and where I bought it. I've been very good about keeping the pictures of fabric file up to date as I do it on the day I buy the fabric. It's nice to see what I have without going through all of my cupboards, when I am looking to see if I have a certain fabric/or color of fabric. If the fabric is bought for a planned quilt, then I note it there.
Then on a spreadsheet in excel, I have a file where I list the fabric by name/color and in the line across I list how big each piece is or how much yardage it is. (this list is a little off and on) If I don't mark it right when I use it, I do a scattered inventory check when I'm having a bored day and bring my spreadsheet file up to date. (for example, for one fabric, I might have a 2 yd piece, a 8 yard bolt, and a 1/2 yard piece - and I list it like that separately across the line)
I also keep a listing of the books/patterns I have. I can't tell you how many times I have bought duplicates (mostly in my regular fiction books, but once in a while with a pattern book) I have a file where I list all of the books I have - first by what kind, fiction, quilting, etc. then by author and lastly by name of the book. I do this when ever I buy a book.
I also put together a list of all the quilting rulers I have. Both the cutting rulers and the quilting rulers, so I know what I have (especially the quilting rulers). These are just in a notebook, with a picture of the ruler (quilting rulers) so I know what shapes/designs I have and just a listing of the cutting rulers and sizes of them.
I really need to do this for my tools. I know I buy duplicates (seam rippers, awls, etc)
I don't do this for my thread, but since I only use three basic colors, I usually know how much I have. And I usually wait and buy in a lot of a dozen or so when there is a sale on and I am getting low.
Nor do I do this with my batting - which I buy on a roll, and sometime in packages if they are having a big markdown. I know when I use it if I'm down to the stage I need to reorder.
... I just like to be organized
ETA: when I plan a quilt or buy for a quilt I want to make, I keep all of the fabric together and put a sheet of paper with it, listing what it is for (I have shelves for this) then when I'm ready to start the quilt, I don't need to search to see if I still have all of the fabric. Otherwise, the fabric not on these shelves are free to use as desired.