Dear Miss Snark,
At what point in the query process should the author disclose prior representation of a manuscript? I fear it'd be the kiss of death on an initial query, yet waiting until the agent says they wish to represent the author seems unethical.
If you let me invest hours of my time reading something and THEN disclose it's been shopped till it dropped, I'm going to auction off your nether regions on ebay...before I remove Killer Yapp's delicate pink snout from your left ankle.
If you've been represented before you gotta tell me. Early. You don't have to give me all the prurient details but the more along the lines of "my former agent was abducted by aliens and left publishing for a lucrative speaking career" the better. In other words, I'm not going to be all that eager to look at something that's made the rounds and hasn't sold.
On the other hand I just came from a dinner engagement with a colleague where we toasted her success just today in selling a book that had been shopped extensively by someone else. She had a few tricks up her sleeve that no one else did. The key however was the author was SMART and revealed all, early. My pal knew the stakes and was willing to get fired up.
I think I compared this to dating after a divorce. I don't want to hear about your ex wife and how she took you to the cleaners but I do want to know she left you with six kids and psychotic golden retriever.