Hard for me to imagine there is much interest in Robert Jr, but I would move him is a heartbeat.
The Mets best chips are likely the pitching. Where Minter, Reley, Brazoban, and Weaver could likely bring decent returns. I hate the annual tearing down of the bullpen and trying to rebuild it again every offseason, but this is where the Mets have value as sellers.
Holmes would be worth resigning, but is otherwise our best trading chip of the starters. Peralta should be moved, though I don't see him bringing back much other than mid level prospect.
I don't see Bichette as a bust. He has acclimated to third as well as should have been hoped. He got off to a horrendously slow start. His line since June 1st is .307/.344/.464/.808. Even with his slow start he is hitting .263/.337/.400/.737 with RISP for the season. Is he overpaid, yes I think so. However, he is not the reason for the Mets lack of offense. If he can be traded, for a solid prospect then fine, but I doubt it.
We are stuck with Semien, Baty, and Vientos. I would move Polanco if at all possible, but I don't see that happening.
Alvarez could potentially bring back a decent return from a contender in need of catching, but then either Torrens because your starter or the Mets need to fill the catching position in the off season.
If the Mets are to acquire players that can help contribute next year, they will almost assuredly have to dismantle much of the bullpen.
If being sellers means we trade for a prospects that are 2-3 years away, then I really don't see the point of trading usable pieces that are under contract for next season.