So the lightsaber being found on Bespin and then sold, going from one owner to another until someone close to the Jedi acquires it is too far fetched? It going from Bespin to Jakku without anyone being interested in it at all would be an even bigger plot convienence than anything in the actual movie.
Rey isn’t a Jedi plus as i have already said Luke has bought a new saber from the shop for Jedi so why would he actually give a fuck about his old one that he assumes is lost.