The market platform has several significant disadvantages:

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Lack of moderation of communities, you still have to choose a quality platform yourself.
There is no way to compare sites by cost and coverage.
The platform primarily selects publics with coverage, most of which have entertainment content, which is not suitable for all categories of online stores.
You can't choose a specific community, only from those that the platform has selected.
If your online store does not work with a wide audience, it is better to negotiate directly with the administration of suitable communities.

Posting through community administrators

In this case, you will have to search for a suitable site manually. With this selection, the chance of finding a quality community for placement is much higher.

Main requirements for the site:

appropriate topic;
live audience without bots (at least check the avatars visually);
feedback from subscribers (whether they comment and like publications);
how many advertising posts are there in the community (by the way, VKontakte rules allow up to 5 advertising posts per day and only 3 of them can be published outside the market platform);
statistics on coverage, gender and age (you will need to request it from the administrators).
This approach has its downsides. Firstly, advertising database direct placement may cost more than through a market platform. Secondly, you have to rely on the administration's honesty in terms of the statistics provided, compliance with agreements on the placement time, etc.

Third-party exchanges for finding communities

Another option for automated community selection. The advantages include a large number of filters for selection, but there are also disadvantages:

additional fee for site search;
inaccurate data about communities, including those that no longer exist.
In thematic communities, you can advertise your public (and thus gain subscribers) or specific products from the catalog. However, remember that advertising products to a cold audience is less effective and works most often in relation to consumer goods.