I agree with advocate Dimple Jindal regarding 'possession'.
Proving physical possession of vacant land is often difficult in legal proceedings often, as both contending parties produce a set of documents, which can be gone into in detail by a court only in protracted and costly legal proceedings of a 'declaratory suit' or 'eviction suit'. If possession is provable, then an easy 'injunction' suit can be filed.
To reduce your issue in legal tems, you are the owner of a property having purchased the same on a date prior to the date of purchase of the same property by another person. The three main issues that would arise would be (a) Who executed the sale deed in both cases and whether that person had a clear title to be passed on to the purchaser; (b) whether possession was handed over to the purchaser (c) whether the descripttion of the property in both cases is exactly the same including the boundaries.
Another important thing is that whether you have submitted copy of your sale deed to the revenue authority, panchayat/municipality/corporation, got the 'patta'/ khata transferred in your name and paid the property taxes regularly. If not done so far, please do so immediately.
You may have to engage a local lawyer in the case of litigation. As the case is not a complicated one no hi fi lawyer may beneeded. One with about 5 years of practice with good presence of mind will sufice.