Give them away - Donate them - encourage someone that has no helmet to consider wearing one.
IF THE HELMET IS UNDAMAGED & IN OTHERWISE DECENT SHAPE, IT IS JUST ABSOLUTELY STUPID TO TOSS IT IN THE TRASH CAN!
I do understand that some of you may be proprietors of bicycle store(s) or perhaps you are an employee of such a local bike shop, so you might feel that it is best to have them tossed so it doesn't impact potential sales. Look at it this way, anyone getting excited about riding a bicycle or someone returning to casual riding of a bicycle for exercise/recreation is always a great thing, even if those folks revive a 45 year old garage sale bike, or purchase a $150 Wal-Mart/Target special, because the more folks rollin, rollin, rolin........will always inspire other doofusses, because naturally some folks wait for others to do it and they gauge whether it is an "approved behavior that "other folks" see as acceptable. Yeah, you have plenty of people that are afraid to do something because the need to be sure that it is something that............well they just have to have to have confirmation from their neighbors/friends/relatives/peers that it is "approved behavior". These were the people that you remember as kids that were hesitant to get into the pool..............they would check the water with their toes to see if the temperature was acceptable, and they would wait for hours watching others have fun in the pool, because they were still somewhat unsure. Those people eventually do dive in and enjoy it such that they don't want to get out of the pool at closing time. I imagine that there are some folks like that when it comes to considering getting on a bike for the first time since they last rode one at age 14, some fifty years ago. There are always some of them, no matter the age, that have that 'Oh, I dunnnO , its been so long, I dunno, I don't have any sunglasses, I don't have a helmet, I don't have the time, I am too outta shape to ride, I don't have a pump to air up the tires,..............just negative, excuses for not trying to do it...........................It isn't really those, it boils down to they don't want to appear stupid and get laughed at, although nobody will, in their minds, they believe that might be the case, so they miss out on fun that might also do them some good.
This is where giving away an old rebuilt Schwinn/Raleigh/Columbia/Peugeot/Huffy/Murray...whatever it might be, to a neighbor, friend, relative, etc with a decent functional Helmet could be just the jumpstart that this person might need. Obviously, you'd have to help them get started, riding slow with them, at an area that is perfect for those no automobile traffic, slow rides, without hills, enjoying the scenery and nice weather------------if you get them moving and they enjoy such an experience, well, there is a chance that they might continue to do the short, slow-ride thing, when the weather is nice.
You have to be realistic, and be careful not to overestimate what someone may be able to do because you're a fit triathlete that can do anything...............you may just have to do the 8mph ride thing with them in the park on flat roads.
Give the Helmet away, or donate it to Salvation Army or Goodwill. Sure, you might have somebody that is a professional-shopper, FLIPPER, that goes to the Goodwill/Salvation Army daily to find merchandise to re-sell on their EBAY store or Craigs' listings. In that case, well at least, GOODWILL, & SALV ARMY received some cash to continue their good works. The flipping thing happens mostly with nearly new, top-quality name brand items, and it could happen with $2 Salvation Army helmet sale if that helmet is a high-end one that the Flipper recognizes as such, otherwise no body is likely to attempt to flip a $2 helmet unless they have a weekend booth at a local flea mkt and regularly sell used bicycles and such.
Don't toss an otherwise decent, useable Helmet into the trash can.
Someone may benefit from wearing it.
There are never enough folks using helmets, as the belief that well, you know I just don't need one because I have never needed one and I'm only riding safely and slowly. Let us hope that no one finds out the hard-way that oops, oh shoot, you know what, I should have worn a helmet. Let's hope that no one ever has their relatives and friends gathered together saying "oh dang, I just can't believe Chuck is gone, I saw him on Tuesday and he appeared to be as fit as Chuck Norris, wow, its so sad that he's dead because of falling off a bicycle, we're gonna miss ya Chuck, sure wish you were wearing a helmet!"