He didn't have a market before we traded for him and he doesn't have a market now.
Fans always talk about "showcasing" players to "improve their trade value", which usually means giving a dude lots of undeserved PT so he can rack up some meaningless counting stats. NBA GMs make mistakes now and then but they're not THAT easily fooled. Everyone knows who John Collins is.
Collins' value bottomed out in Atlanta, but not because he couldn't make perimeter shots for a while with his wonky finger. It happened because he's not good enough to play in the kind of role he needs to maximize his skills. He needs a super spaced interior, constant involvement in high pick and rolls, rim protection around him, and a genius of a PG to get him the ball when he dives. He was always undersized for a "big", but for a few years he was also hyper athletic because he was juicing. That helped.
In the modern NBA game, a player who needs to roam the interior like that to function well also has to be much more of a hub type. The straight line, pogo stick rim runner still exists, but that's not a $25 million per year player... or an automatic starter. On the Jazz, Collins has looked lost because he still doesn't have his old "comfort zone" role. He's trying to be an opportunistic scorer and spending much more time on the weak side, but that's just not him. And almost nobody's trading for Collins to play like that while dhe makes 25 per year.
The trouble is, he's not the old alley-oop monster anymore either. He's lost just enough athleticism for it to matter. So what is he? Ideally, the ultimate luxury bench player who you play when the matchup is exactly right. The market is very small for guys like that.