To begin with, I recommend that you make a request for an extract from the State Register. From the document, you can find out who the owner is, whether there is an encumbrance. If everything is fine with the living space, you can sign a preliminary contract at a notary.
Buyer's documents:
passport (for minors – birth certificate);
marriage certificate or divorce certificate;
written consent of the husband/wife — if both participate in the agreement, this document may not be drawn up;
permission of guardianship authorities - if the apartment is bought for a child.
Seller's documents:
legal documents;
extract from the Register of Property Rights;
passport, identification code;
written consent of the husband (wife);
technical passport for the apartment;
marriage certificate or divorce certificate;
permission of guardianship authorities - if there are minor co-owners;
certificate of family composition;
opinion on real estate valuation;
refusal of the co-owner from the priority right to purchase a share of the apartment - if part of the real estate is sold;
a certificate on the absence of arrears from utility payments.