If you end up with leftover parts from a project - take them back! Even months or years later, most stores will give you a refund or store credit for merchandise in original condition in its original packaging.
In preparation for our move, we are cleaning out the garage and selling off lots of stuff. Like most men, I have a lot of "parts" on my workbench, from jars of nails and screws, to boxes of things bought at home improvement stores, left over from various repair jobs.
These"things" include outlet covers, outlets, outlet boxes, circuit breakers, PVC pipe fittings, copper pipe fittings, electrical conduit boxes, and other accessories, parts and stuff bought for a project, that are now left over or were never used.
Many folks will pack and ship this stuff to their new home. That's dumb. Others will simply throw the stuff out. That's at least eliminating clutter. Some will sell it at a garage sale for pennies on the dollar. That's slightly better - at least you get rid of the clutter and get some money for it.
But if you have things that are in original condition and have their original bar codes on them, well, take them back! You will get, at the very least, store credit for the full purchase price of the item, or cash back (if it is less than $10) or credit back to your credit card.
Most big box stores have generous return policies, as they want to encourage you to buy things for your projects, knowing that you can take back what doesn't fit or work. If you leave the store not buying something, well, they know you are not likely to come back.
And if you are bringing back salable merchandise in usable form, well, you aren't cheating them in any way - they restock it on the shelves and it gets re-sold.
I found a box of PVC conduit boxes that have been sitting around for some reason. These are not cheap. Neither are PVC plumbing parts, if you add them up over time. I found I had about $56 in credit at Lowes and another $25 at Home Depot. Since moving entails completing a number of "punch list" jobs anyway, these returns paid for most of the repairs I needed to make on my home.
Get in the habit of taking things back. Storing stuff you bought and cannot use makes no sense whatsoever, particularly when you can take it back for a full refund.