Is tax liability for purchase from composition

No, RCM would not be applicable.

  However a registered dealer should avoid buying from composite dealer, as the tax paid @ 1%, would form part of his cost as credit of the same would not be available.

@ Bhargav Verma A composite dealer doesnot issue taxable invoice, but he issues a Bill of Supply for the product sold. In case of a trader, he is liable to Pay GST @ 1% on turnover. Hence whenever he sells his goods at a certain price, it always includes the GST of 1%. And this gst, cannot be claimed as credit by a registered dealer, when it purchases from composite dealer.

Again re-iterating that RCM for supply of Goods and Services  is as per Sec 9(3) and 9(4) of the GST Act. Sec 9(3) is specifically for the notified category of person(s) only and Sec 9(4) is for URD(Un registered delears).

Dealers registered under Composition scheme is not fall under Sec 9(4). As a person is already registred under Composition Schme and having valid GSTIN number.

Now upon examining Sec 9(3) covers category for person as the Government may, on the recommendations of the Council, by notification,specify categories of supply of goods or services or both, the tax on which shall be paid on reverse charge basis by the recipient of such goods or services or both and all the provisions of this Act shall apply to such recipient as if he is the person liable for paying the tax in relation to the supply of such goods or services or both.

Till now for material purchase,  class of person already notified by Govt. on the recommendations of the Council by way of Notifications 04/2017 dtd. 28/06/2017. Hence question of RCM on purchase fof material from Composition dealer is not arise.

In nutshell and once agian reiterating that No RCM on purchase from Composition dealer.

Hope, this may be useful and serve your purpose.