How I think the SOTR movie will change the Mockingjay pin situation
maysilee originally owned the pin in both the books and movie (now, as seen in the trailers).
it gets returned to the donners, and merrilee insists on keeping it. but the longer she has it, and the more her grief comes and goes in waves (as would be realistic), the harder it is for her to keep it. so she tries hiding it away, tries to throw it out but changes her mind last second. for years this goes on in the background. she eventually gets married and has madge.
but since the first hunger games film omitted madge, here is how i think it'd end up in the market where katniss in the film gets it.
merrilee as an adult is said to be struggling with health issues and isnt like how she used to be. so because of her mental health, she finally was able to throw out the pin. but it ends up in the market because it's simple: the pin is made of valuable material and its beautifully made, so why not sell it in the market?
eventually the old woman in the market takes it with her other collection of things in the box, where katniss picks it up because she recognizes the mockingjay bird.
the rest is history.
i think to tie it into the movie, haymitch would be given the pin by someone in the capitol or maybe he finds it left behind before he goes home, and he takes it back home with him, and gives it to the donners after the funeral (the scene in the trailer where merilee looks pissed; my headcanon would be haymitch accidentally called her maysilee, she looks pissed, then haymitch snaps out of it momentarily to hand her the pin and then walk away, upset).