I'm not disagreeing that United are a bad seller but I think there's some confirmation bias occulting some terrible sells that happen every market (this year Chelsea has a few of them; Aubemayang, Ziyech, Lukaku though the latter had a hand in sabotaging it) and it also reflects the erratic & underwhelming results of the last decade.
United only typically sell down to smaller clubs, a lot of said players are labeled as literal deadwood by many of us why would prospective buyers think different ? They're relatively old, on big wages and have not shown enough at United and grew stale.
Going by transfer gossip and what we may hear off : Not a lot of buyers lining up either. Henderson has Forest, Elanga was linked to Everton (but they signed someone else), Fred a Saudi offer. Maguire maybe West Ham. Doesn't help drive prices up. Maybe that's partly a failing by the club.
Elanga is the only one (with McT) feeling like he has some untapped potential.