I wanted to clarify this tweet a bit. So, as you might know, I'm an enthusiast. Sometimes when I'm super excited about a new novel that crosses my desk, I might get second reads or talk to agency colleagues about it before I even sign it. If I know it is up some editor's alley, I might tease them with it -- "I'm about to offer on an aweeeesome super-secret new space opera with dragons... you want to be on the list when I go out with it?" Or "ooooh have you seen this website, what do you think?"*
There's a difference between being a fan of something, having enthusiasm and seeing if other people share that enthusiasm, and actually submitting a person's intellectual property for publication and acting on their behalf when they haven't given you permission to do so.
If you query an agent with your novel** and they tell you they're going to "shop your manuscript around to editors"... and that IF they get an offer, THEN they'll sign you up*** (but they won't otherwise?):
HUGE RED FLAG. THIS IS PROBLEMATIC.
* If they shop your manuscript, and you aren't their actual client, how do you know what they are doing, who they are talking to, what they are saying on your behalf?
* If they shop your manuscript and end up with an offer... it will probably not be the best offer. Why? Because this type of schmagent is unlikely to have primo publishing connections. There are times that going with a small press or an e-book only press or a no-advance press or a start-up press might be a fine idea... but do you really want that to be your ONLY option?
* If they shop your manuscript, and end up never getting an offer and never signing you (which is quite likely, since they will have no particular pressing interest in trying hard, since you aren't a client) -- then let's say you get another agent? A real one? ALL THOSE BRIDGES ARE BURNED. Your new agent won't be able to send your manuscript to any of the editors that already declined it. Assuming you can even find out who those editors were.
An agent isn't just a person who likes books and puts a website up. They SHOULD have a ton of contacts in the publishing industry... and when they talk to those contacts, they REPRESENT YOU. That means they are meant to be acting on your behalf and speaking for you. (Not to mention they have access to your finances and potentially your financial future and career!) -- you definitely don't want somebody you don't trust in that position. Do your homework.
This isn't a game, or a joke. Don't treat it that way. And don't let anyone else treat it that way, either.
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* Mind you, this wouldn't affect MY feelings about it. If somebody was like "ew dragons are dumb" -- I'd think "Oh, they are stupid", not "Oh, I am wrong."
** ETA: I'm talking specifically about NOVELS here, and note that my expertise is in kids and YA. Pop culture nonfiction or other works you sell on proposal might be different, I have no idea. And as some point out... publishers themselves may do some "pre-shopping" to their retail clients when deciding what to acquire. Sadly, there's nothing much you can do about that.
*** ETA ETA: I'm also not talking about people who are legit agents you've agreed will represent you on a "handshake" basis but who don't have a formal written agency agreement, or who only make a formal written agency agreement when money is going to change hands. While this isn't how my agency works, I know some legit agencies that don't have a formal "agreement" per se -- but they will ASK YOU before they start REPPING YOU!