First, as I'm new to this forum, let me say hello and thank you for listening. A strange question: Is there any such thing as an unmarketable concept? Example: I've written a middle-grade SF novel set in the 1950s. It's set up to pay homage to the youth-targeted films of that period, many of which were SF. The protags are typical 50s kids who discover a monster roaming the hills near their small town. There's more to it, of course, but you get the idea. The novel is on the query train now. The trouble is, no one seems excited about the premise. One person told me that historical sci-fi (aside from Steam Punk) is almost an oxymoron and thereby unsellable. What are your thoughts?
Not really. There are people out there with the same interests as you and who want to read something like this. And there's no problem with the idea you have. The quality of writing trumps all, so don't worry too much about what one person says. You can also search other books that are similar to yours and see how well they are doing and what kind of reviews they are generating from readers/critics.
A story concept means nothing. I can tell you now, it has been done before. What matters is how you write it, the characterization, the flow, the imagery, all of it. There's no benefit in asking what other people think of the concept! They'll either say,"Sounds great," or, "it sounds like a ripoff of..." If the idea stirs you, write it. Then ask people what they think of the final story. After they tell you what they don't like about it, revise it, usually several times, until you're happy with it or until you throw up your hands and say the hell with it. Please read this thread about What is Plot Creation and Development? Forget about the marketability of a concept.
Are you pitching it as a fifties story in your query? I could see a problem with it, but that’s only because it doesn’t sound very exciting. But as the others have said, how you write the story will determine if it is any good. However, you should consider another angle in your query letters.
Hello, welcome, and congratulations on having a novel manuscript "on the query train." My thoughts about what you were told is that agents, if anyone, are exactly the folks who ought to know something about what's "unsellable." That may only mean, though, that you're marketing to the wrong agents or maybe your query needs rethinking. It also is well worth considering the possibility (even likelihood) that in a genre that fits the young market you're targeting there are premises that work in that market and others that just don't (I'm thinking specifically of your young contemporary readers' capacity to identify with a '50s main character and/or setting). I think if you're writing genre work, you do need to pay attention what appeals to its audience and what doesn't. Good luck with it!
You might have to exaggerate the 50s aspect for those who know nothing about the era, e.g. like the 'Back to the Future' films. This is purely my opinion of course, but I'd say if young people don't know anything about the era, you're going to have to offer some humour or something for them to engage with. A super-serious style wouldn't interest me so much, but then it's hard to say without seeing something.
