you have to file civil proceedings too against the sellers . make purchasers a party to the proceedings .
iif in criminal complaint you had made allegations of criminal conspiracy against sellers and purchaser then court would have directed police to investigate their role . it all depends upon averments made in criminal complaint filed by you . in complaint it has to be specifically mentioned that on your plot fencing was erected , gate installed , name plate was displayed and that purchaser was fully aware that the sellers were not owners of the plot