As a trader, today you are liable for registration under State VAT if your turnover exceeds the prescribed threshold limit. You are also liable for registration under Central Excise if you trade in Excisable goods. You are allowed to avail of input VAT credit and set it off against the output tax liability. However, the excise duty you pay will not be available as credit, and it will be added to the cost of the product, and subsequently passed on to the buyer.
In the GST regime, it does not matter if you are a manufacturer, a trader or a service provider. The input tax credit is available across the supply chain until it reaches the end customer.