I queried my first novel a few years ago. Several agents wanted it, and I signed with one. The agent then was not able to sell the book. I parted ways with that agent due to some concerning behavior that had nothing to do with the MS (I was always aware it would be a hard sell).Now I have another book. I'm wondering how it would be perceived if I sent this novel out to some of the other agents whose offers of representation I had turned down. Should I mention that they had previously extended offers, or not say anything?
They will not have forgotten.
Trust me, I remember every single person who says no to an offer.
Your problem is not requerying them.
It's not telling them they were second choice.
It's the "concerning behaviour that had nothing to do with the manuscript."
If you part ways with an agent, you must be prepared to explain why you did so, and what went wrong.
To answer your question: if you query agents who previously offered rep, mention it, and say why you're back out looking for a new agent.