You are to pay GST at depreciated value. Depreciated value can be computed by many methods but two methods are very suitable and beneficial to the assessees. These are : Written down method and straight line method. Both are easily available on google. You are to choose according to your suitability and by which you can get the maximum benefit.Thus you can arrive at correct transaction value/assessable value for correct payment of tax as per above method prescribed in accounting principles.
Regarding the rate of GST, GST Tariff Act makes no difference whether it is new or old machinery. Chapter/Heading/HSN remains the same. What benefit you can get that is reduction of value(depreciation) and not any benefit from rate of GST for selling/supply of old machinery.
Now there are different slab rates i.e. 28%, 18 %, 5% Rate will be applicable as per type of machinery. Machinery falls under Chapter 84 of GST Tariff Act. Intimate the type of machinery and function/purpose of machinery for correct GST rate.