Dear Miss Snark,
I was recently dumped by my agent after several years with no sales. One of my manuscripts has only been seen by 3-4 houses, and I may not be willing to give up on it yet. You've said before that you needn't mention prior relationships with other agents in a query, and that in fact this may be a negative since agent-hopping is not looked upon kindly.
My questions are: Is it looked upon even less kindly if the hopping wasn't my choice? Would another agent care that this well-respected agent was enthusiastic about my work (well, you know, once)? If I don't mention it in the query, when is the right time to tell a prospective agent that a few houses have already seen this?
You DO need to mention that your work has been shopped and you need to mention it early on. One of the things I think about very early is "can I sell this?" and if a project has already been seen by three of my favorite editors I'm less likely to take it on.
I'm not much of a fan of second-go-round projects. If you've got something new, I'd much rather look at that and take this used project for a backup/second book.
And you can just say you and your agent parted amicablly after a period of years. Chances are if we progress to actually talking about hanging out together I'll ask what happened, but at the query stage, I just want to know it happened, not details.