How can user place an adverts in a separate block?

I am not sure what do you mean by “place an ad in a separate block”?

I mean that user could have an opportunity to publish an ad not only in common list, but in a block of sidebar, for example

We do not have a specific feature for this kind of functionality, but if you allow users to post paid featured Ads, then in the sidebar you can add a Classifieds List block and configure it to show only the featured ads.

How to add a Classifieds List block and configure it to show only the featured ads in the sidebar?

Go to wp-admin / Appearance / Widgets panel add new Classifieds List widget (select post type and form scheme and click Apply), scroll to the “Featured Ads” section and in the “Show on the ads list” select “Featured Ads”.

This will work of course only if your theme is using block based widgets.

This works, thank you! But how to make a tariff of 15 ads and only one of them featured?

We do not really have such feature i am afraid, depending on what you are trying to do maybe having 2 Classifieds List blocks will work for you (one with 1 featured ad and one with 14 normal ads).