Can someone help me check these two sentences for grammar?

Neither version is really optimal, in my opinion.

Agglomeration isn’t the right word for a group of people. What are you trying to say with the word? Are you speaking of number, interest groupings, variety, or what?

Conducive is used incorrectly. Something is conducive to the accomplishment or achievement of something if it enhances conditions that make it easier or more likely. Something might be conducive to serving niche markets but not to the markets themselves. A thing must be conducive to an action or state and not to a concrete noun.

“Medium” is the singular, and “media” is its plural form. You should speak of printed media (or print media), and not of printed mediums. “Mediums” is reserved for people who claim psychic powers.

I have almost invariably found that a sentence with fuzzy grammar and usage is also a sentence with a fuzzy thought behind it. Let’s have a clear, uncomplicated statement of your thought first, and then let’s see how best to express it.

Hmm, thanks. My idea goes like this:

A niche website caters to a group of people with a specialized interest.

Because the Internet stretches across geographical boundaries, it’s easier for an Internet niche website (like, say, Epicurious) to gain popularity than for a niche magazine to gain popularity. There is a larger group of people in the United States interested in gourmet cooking than there is in, say, one state or region.

By “agglomeration,” I’m trying to say that the “mass” of interest in a specialized topic across the whole nation is larger than the “mass” of specialized interest on that topic within just one state.

Because of that, if someone wanted to set up a news outlet that catered specifically to news about gourmet cooking, it would be cheaper and more likely to succeed if it were created on the Internet, rather than in print. (Also very much due to the lower set-up and delivery costs of online vs print, which I mention earlier in the essay.) That’s what I mean by “conducive” – the Internet is more “conducive” to niche markets than the medium of print.

Does that make any sense at all?

Yes, it does. Maybe you should put it pretty much like that. Your idea is stated far more effectively in your explanation than in your original sentence.

Instead of “agglomeration,” you might speak of a population or of the size of a prospective audience.

Having a widespread prospective audience might not be the obstacle to popularity, though, just to comment on the thought. The mail goes all over the U.S. But it might be an obstacle to promotion—to reaching that prospective audience and attracting them as subscribers or customers.

Something still can’t be conducive to a market. A market is not an action or state. Try substituting the definition in the sentence and you’ll see what I mean:

The Internet is more tending to produce to niche markets than the medium of print.
The Internet is more conducing to niche markets than the medium of print.
The Internet is more contributive to niche markets than the medium of print.
The Internet is more helpful to niche markets than the medium of print.
The Internet is more favorable to niche markets than the medium of print.

(You should be able to do this with any well-formed definition; it should always be substitutable for the term, though sometimes requiring slight rearrangement of the expression.)

You are not talking about being more helpful or favorable to the market. You are talking about be more helpful or favorable to reaching or serving or attracting or developing the market, correct? So you need another word in there.

Simple straightforward sentences are best. Never try to impress your audience with big words especially if you don’t fully understand them.
What remains unclear in the examples you gave is the rationale of your argument.

The huge potential audience on the Internet combined with search mechanisms that allow one to sift content by interest makes it ideally suited to niche marketing.

Thank you guys so much!