Sales tax element in the thing which you sell? Like you might buy a lamp for interior decoration and sell to customer on that no st is applicable. But your interior design charges over and above sale of lamp is taxable to st but not as works contract. Supposing you but lamp and work on that lamp designing such lamp and then you sell the same lamp then it is works contract. Or even place the lamp by constructing fixtures and other things then it is works contract.
In short where there is only selection and placing of goods involved it is not works contract but where there is constructions like building fixtures for mounting of certain goods etc is works contract.