Funny enough! There is a donation/disposal bin for bear-spray at BZN (the primary airport you fly to visit yellowstone).

The funny part is, this problem has already been solved by the thru-hiking and backpacking community. We call them "hiker boxes". But for them to work, it requires an agreement within the community on some unspoken rules. Things like take only what you need, try to leave more than you take, and don't abuse the system or everybody loses. Hikers actually enjoy leaving things behind that they hope will solve some immediate need for someone else, even if they never get to meet that person.

Looking out for one another in ways like this is mostly just baked into the subculture and that's one reason hiker boxes work (the other being you just don't want to carry extra weight). As someone who has backpacked through Yellowstone, I'm not sure it would work for the tourist culture there.

It really shouldn't be hard. As you're walking out, just hand it to someone walking in. Even for our State Fair with the coupon racket that is used, I've been handed a string of tickets walking to the gate from someone that bought more than they could use. Funny timing though, I had already pre-purchased tickets that I didn't even use. On exiting, I gave away what I was given plus what I didn't use.

The fact that there's not a stand/bin with a "need one, take one" type of sign is kind of head scratcher

The rental cost is nearly the same cost as buying a can, or at least it was when I was there.

No, bear spray is the same as pepper spray, except highly pressurized to shoot a longer distance (10m vs 2m). Bear spray has the same active ingredient as pepper spray (capsicum).

Bear spray works as well on humans as it does on bears.

No, it's like a super pepper spray. You definitely don't want to spray it on yourself.

Yes