I've written a mystery about a child that is abducted - she is rescued in the end. Recently, I was given critiques from several writers - one of them suggested that the subject matter is too tough of a sell. What is your opinion on this (other than if it is perfectly written it will sell)?
What the hell does your writer friend know about selling? If she's published she either sold it herself so her opinion is drawn from her own limited experience, OR her agent sold it and your friend didn't even do that much selling.
And if she's not published, her opinion is based on her own difficulty getting something published.
She's probably basing her opinion on something she's read or heard: the publishing industry is FAMOUS for agents and editors bemoaning the state of the industry.
I've ranted about this before, here it is again: be choosy who you listen to. Your writer friends mostly don't know much about selling work to publishers; Miss Snark on the other hand doesn't know much about character development (and you can take that any way you want to).
Anyone who says "this subject matter is overused" is offering an opinion. Opinions, unless you are a judge, don't have the force of law.
And really, everything is a tough sell.
Focus on that.