1. Since the current owner is the owner of the apartment, having paid the entire amount towards the sale proceedings to the developer and got it registered in his name from the developer,. you have to enter into an Agreement of Sale, with the current owner only.
2. Before buying the apartment, you have to ensure that, whether any money has to be paid to the developer or any NOC has to be obtained by the current owner from the developer or the association, for resale of the apartment.
3. You can meet the developer, along with the current owner, to obtain the required information.
4. Likely that the developer may charge some amount for resale of the apartment.
5. You can get the property papers verified by a Lawyer, before going ahead.