My reply is from a very different vein, since this was something people had worried over at our own food/supply pantry! What I asked volunteers who were concerned was: Why is it a problem if people end up selling the items? What negative impact does it have? Who does it harm?
For us, all the items we stock in our pantry are donated items that the shelter can't use. We already have them, and if we don't find someone else who can use them, they are headed for a landfill. If the secondhand non-adjustable dog harness goes to a pantry visitor who sells it to his neighbor for $5, is that impacting someone negatively? If the neighbor couldn't afford that harness, we probably will get another donated soon that we'll give away for free.
For us, we have the additional factor that much of our county is rural, so access to pet supplies (and even our programs) is limited by people's ability to travel. If someone takes ten leashes and gives them to their friends who don't drive, that's probably ultimately a benefit to us. (After all, that same person who didn't have a leash and a car is going to have a hard time getting to the other side of the county to pick up their stray dog.)
Obviously, this approach might not work if you are soliciting funding/supplies specifically for the program, but from the sound of your post, these are leftover items in need of a home. If they go through one more set of hands before going into use, is that worse than continuing to hang onto and store them?
If you really want to distribute them for the use of a particular pet, I think giving them to the owner at a time that they have the pet with them is the most successful option. Does your local Health Department host any free Rabies vaccine clinics that you can tag along with? Or is there a low-cost S/N program where you can table and hand out supplies?
Just my two cents from a different viewpoint!