Purchase of parking lot

Hello, We are going to purchase a parking lot from a fellow resident in our complex. Our concerns are as follows: 1) Whether such a sale is allowed by law and whether the sale deed can be registered? 2) If there is an internal sales agreement between both the parties and the same is not registered,whether my rights on parking lot will still be protected? 3) Do i need to involve the builder because both the parties have got the possession of their respective flats and parking lots. 4) Is a sale agreement/deed enough or do i need to more documentation? Any guidance will be highly appreciated. Thanks & regards