I'm continually getting emails that say something like: "DL...last (week, month, year) you had some buttons in your shop. They were (round, square, purple, orange) and I loved them. But now I can't find them. Do you have any more?"
Sadly, many of the buttons I carry are not available in large quantities, and are not around for long. A simple fact of retail is that slower-moving items stop being offered. Other times the resources (like 14k gold or sterling silver) used to make a particular button drive the prices up too high to be readily affordable, or the artisan simply moves on and decides to focus on a new direction. Either way, the quantities are limited, and once they're gone, they're gone.
I see having a button stash much the same way as having a fabric stash or a collection of art supplies on hand. You may not have a particular use for them when you see them, but if you like them, buy them up -you never know when inspiration will strike, when a particular project will just cry for an item seen somewhere last month or when you'll simply decide the time is right to use them up. And if you are working on a project that calls for three or four, buy an extra, because you may never be able to find another that is a perfect match later.
OTOH, when replacing the buttons on a garment, if the old ones are unique or handmade, keep them! You just may decide you need them later for another garment or they may prove the perfect finishing touch for a scrapbook page, handbag, or custom jewelry item.... Buttons have so many uses and purposes that I firmly believe you -or a friend - will most likely find something, somewhere they will be perfect for.
Life is short...buy the buttons, the incredible fabric, the art supplies, the shoes or other items you love. You may not have another chance, and even if you can't picture a project or use right away, you'll know that when the right idea hatches, you'll have the most beautiful supplies to make it with right at hand.