I am about to start marketing methyltriethoxysilane locally, though it feels my lack of expertise with this product is a bad starting point. I would really appreciate a piece of advice or useful info from someone more experienced with this chemical. Thanks in advance.

To sell to makers of silicone caulks and so on? Seems a very bizarre product to market 'locally.' Not what you average consumer is looking for. It's for large commercial sales shipped world wide, no?
Can you please explain how this makes sense?

I represent local Ukrainian chemical distributor company. We do not have any big silane consumers, so "large commercial sales" is not for our country.

I received two inquiries for MTES, both of them are from pharmaceutical industry. So now I try to understand what is purpose of this product for I could promote it further.

Thank you.

It has many purposes across different industries, and it's purpose can be found by reading many google searches, which I'm sure you know.
Pharma, agriculture, dyestuffs, etc.
You don't say if the inquiries were from within your country or not.
I wouldn't even begin to try to learn all the answers to questions about the product, which could be anything from what is allowed by law in different industries, to how pure a chemical has to be, to environmental concerns, to import/export laws and taxes.
We who volunteer on this site can certainly look up some helpful links for you, but I'm sure you have done that, and know far more already than anyone here.
Your best bet is spending several hours reading one site after another, and going deeper into the subject.
For example - is yours made in China, and how highly regarded is it for any specific industry?
Before you can market it, you have to know all about it.