I've often considered the idea. I LOVE the idea, as I hate querying... But I almost feel like it would be false advertising... your query is supposed to showcase "your" writing skills so the editor can decide if they want to hire you or not.
Also, querying is so emotionally harrowing (for me) to begin with, I'd wonder if the story was rejected because of my idea or my employee's writing. And... finally...
If someone is good enough that I could pay them to write queries and sell my stories to top markets... why on earth wouldn't they just sell their own stories to those markets? LOL Ideas really are a dime a dozen, it's what you do with them that counts.
Market research, though, that's another story! You could hire a VA (Virtual Assistant) to do that for you, and they aren't very expensive. (relatively speaking). Or a college kid dying to learn the ropes of freelancing. In fact, my niece may be available to do market research. (We've talked about it, and I may hire her later in the year for it... either that or babysitting, and since I like playing with my daughter more than I like market research, it makes sense to go that route! LOL)
Dawn