It is a good strategy to try to be diplomatic even when faced with offers that are borderline offensive. It is also a good economic strategy to avoid entering into an offer-counteroffer situation if you think the first offer is way too low. (It creates an [Anchor](https://en.wikipedia.org/wiki/Anchoring_(cognitive_bias))).

I have found that a "thank you very much, but no" *without any arguments or justification* is best in these cases. Said in the friendliest manner possible. So my advice for Tania would be something along the lines of this:

> Thank you for considering my room and making an offer on it. I am, unfortunately, not selling it [right now]. 

If said in person, it should be said with the same tone of voice you would have turning down a cup of tea. As-a-matter-of-fact and with a gratuitous tone. If written, it should be written as *informally* as available. "Hi Oleg" (I know that familiarity, nicknames and honorifics are pretty complicated in Russian, so i'll leave it too you to decide what is familiar enough...)

You may, if courtesy allows it, include the ending *[for 200.000 $]* instead of *[right now]*. That will be a bit more leaning on him to make a new offer - but it is also clearer in your lack of enthusiasm of the offer itself.