So what do you with them when your little one has outgrown them? Many moms sell them on Craigslist, Spots, Diaper Swappers, and more. This is what I did with the majority of our small diapers. I've even sold a bunch that didn't work for our family there. For example, we don't really like the old style bumGenius all-in-one around here, so I sold the couple that I had and replaced them with something we DO like.
When preparing your diapers to be sold, I suggest avoiding terms like "excellent used condition" or the like as those terms are very difficult to understand for most. What is EUC to one person is not necessarily EUC to the next. The best way to list your diapers is to take plenty of pictures in good light (inside and out), mention all flaws you see with the diapers within your listing, and mention whether your house has pets, a smoker, etc in it as there are many people who have allergies to these things. I also like to include the detergent I use on my diapers for the same reason.
If you want to keep your outgrown diapers for any reason, be sure that you store them properly so the elastic, etc isn't ruined in storage. Extreme changes in temperature are a big problem for anything you store, but it's especially bad for diapers. So for the most part, the attic and the garage are out. Moisture is also an issue for diapers as it can cause mold, so this knocks out the basement as well. I've found the best place to store the few diapers and covers we've kept (and I'll likely sell them here soon) is in a bedroom in a sturdy plastic container with the lid on.
One last note on diaper storage: If you've already stored your diapers and are planning to use them soon, be sure to take them a few months ahead of time so that you can check each one. Check the PUL, the elastic, and the closures. Make sure everything still works. Look for stains that may have shown up that you don't remember.