I bought my Red RL Jersey off Alex Lundqvist when he was selling it on PBnation, If he hadn't been selling it there would be no way that I would have it now, I love RL but can't exactly go to their tourneys and ask them for one. Even though it was well worn I was so happy I got it and love to play in it, worth every penny, I wont sell it because it's RL, it's rare and nobody else will have one (players got 1 training jersey each).
More recently got a disruption jersey off ollie in return for a favour. The jersey was probably worth more to me than the favour was to him but I am grateful for it and definately wouldn't sell it.
So in my experience the players should decide what to do with jerseys they have put hard work into earning. Though if there's a guy down the local field that always watches and asks questions or just has mad respect for the pro then it would mean so much more for that pro to just give it to him, the good feeling of seeing someone enjoy what you gave them would be better than money, but I don't think it's bad to just sell it.
Also, im glad the pros do decide to part from their jerseys, playing in kit you can feel proud for owning (even if it's not your team) is important for field confidence and my jerseys have helped me...
win a tourney,
play one of the best teams in the country,
and at a trial, didnt get in but it gave me the confidence to play above my level, and walk away a better player for it.