In general it would be extremely irrational to assume magic items cannot be sold as they are very valuable, very useful and at the prices listed in the DMG, they are dirt cheap. Any NPC with 100 GP on their hands would buy an uncommon from you without a second thought. They would be a complete fool not to. Unless your world is populated only with complete fools, selling a magic item would be extremely easy.
But then, why would a PC sell a magic item for this worthless gold they have piles of already which they can't spend on anything useful?
And you're right back at the problem 1E had by assuming magic items were super rare and couldn't really be bought which went away as later supplements came out, because it really makes no sense. Ever since it's been standard for magic items to be as common as other magic until 5E came along to repeat mistakes.
Just assume magic items can be bought and sold, most tables I've seen here already do and as more supplements come out with magic shops written right in the system will be assuming it does too.