Is this a "red flag"?

It can be, it can not be. I know fo some places that are fine that ask for this and don't want more than a paragraph indicating that you are not sans clue in terms of the market for your genre.

IMO, it's a signal that you should check the agent's credentials very carefully.

Advance-paying commercial publishers don't typically buy first fiction on a proposal, or ask for marketing plans to accompany novels. Wanting a proposal/marketing plan as part of a fiction submission may be a sign that the agent is inexperienced, or that she focuses on extremely small publishers (which sometimes do ask for a marketing plan, because they have such limited publicity budgets that they must rely on their authors as an unpaid sales force).

- Victoria

This will sound arrogant, but it's how I personally feel.
If an agent wants a full marketing plan, I don't submit to them. If I had a complete marketing plan, what the hell would I be paying them for? To me, the agent is supposed to know how to do this.
If I have to write out their proposal to the publisher, doesn't that kinda defeat the purpose of paying them a percentage?

Don't get me wrong, I soooooooooooooo want an agent, and hope to be picked up one day by a traditional publisher, but I'm not going to ask the agent to write my book; why would he/she ask me to write their proposals/marketing plan?

That's like hiring a housekeeper, and then cleaning the house top to bottom before they arrive. And the point is?

Now, I did read somewhere that Nicolas Sparks sent out his query letter to include a target audience his book The Notebook would be best suited for, and why. That actually was his selling tool to present agent. I think that makes sense. Creates interest. But the whole marketing plan/publisher proposal....well, hell, if I could do that, I'd be an agent.

Stop laughing. It could happen.

My agent does not market. She sells my work to publishers and negotiates my contracts. When I first signed with her she wanted to know more about where I visualized my books sitting on the book shelves (which author I was comparable to) and who I visualized my readers were- which is something all writers need to understand. This helped her figure out which house/editor to pitch my work to.
She also wanted to know how involved I saw myself in the promotion of my book.
In the author information sheet from the publisher I had to give a detailed response to questions about my platform and specifically who I thought would be most interested in my book even though I am commercial / women's fiction.
But that was for the publisher. These actually are questions that authors need to think about and if an agent is asking them -- I think it is a way to see how savvy an author is about the business.
Gone is the time that an author can sit back and relax and "let the agent and publisher do their jobs" Astute authors realize that they have to be intimately involved from the outset and have an understanding of platform whether it be fiction or non-fiction.
JMHO

A marketing plan is a standard part of a nonfiction proposal, and proposals are standard for nonfiction. If you have a nonfiction ms. and are unwilling to do a proposal and/or marketing plan, you may have trouble.

For fiction...I'm not saying that your agent won't ask you about your ability to self-promote, or that you shouldn't briefly note in your query letter which authors you think you resemble, or that your publisher won't request marketing information as part of the publication process--after you've signed a contract. I'm just saying that for commercial publishers, a written marketing plan is not a standard part of the submission process for novels, including first novels. If you see this in an agent's guidelines, you need to be careful, because it's frequently a sign of amateurism. If you see it in a publisher's guidelines, it's a clue that the publisher may not have much money for publicity or distribution.

- Victoria

I just play it safe and tell them who I think would read my book, whether I'm willing to travel (on their ticket) and that I work for a newspaper so I can at least get local coverage. Let them do the rest, be it asking specific questions or taking the whale by the tail. After all, at this point I don't even know the questions, so how will I provide the answers?

i think there may also be a difference between the kind of agent who asks for a marketing plan just because they want your input to see if it happens to be any good and actually requires a marketing plan because they want you to do all the work. i'd definitely avoid the latter.