Car Purchaser is not transferring ownership

I (Seller) sold my car no.:which was registered under Palam authority (now Dwarka District zonal code DL09) to (Purchaser) at Faridabad in Rs 75000/- Cash. Regarding this I already informed The MLO, Dwarka(in Past Palam), Dwarka Authority, New Delhi, District Zonal Code- 09 on Email id [deleted] by Email dated 10 January 2018 and 8 February 2018 and also informed to the Insurance Company by Email on dated 10 January 2018 and 8 February 2018. Regarding this I also informed in written to RTO District Zonal code DL09 Dwarka(in past Palam)by speed post dated 9/2/2018 and The Manager United India Insurance Customer care department by speed post dated 9/2/2018. RTO Dwarka Blacklist the car no.:DL - 09 - CZ – 3042 by reason that “Intimation of sale of vehicle on dated 8/1/2018 by Registered Owner.” But Now he remove the blacklist of car without any information given by me regarding this I lodged the complaint to RTO & After that Central public grievance cell against RTO Dwarka Even i written to Delhi police but they give me dairy number & said that there is not any rule for to force the purchaser. Even I request in verbal communication and by letter dated 9 February 2018 by Speed post to Purchaser that please transfer the car your name but still he not transfer the car and on 5the March 2018 Mr Jai Prakash (Purchaser) told me that he sold the car to third party. Even I already hand over them all the papers as required by them to transfer the car in his name Still Purchaser not transfer the car in his name. What Can I do now?