Cash transaction limit for bullion merchants

Hello, I want to know what is the cash transactions limits for bullion merchants. I mean if any merchant buy goods worth Rs. 30000-40000 from 6-7 unregistered person daily and total transaction amount comes around Rs. 2-3 lakhs is it permissible under GST and does he have to pay reverse tax on it and if the same he sold under electronic mode of payment to a registered dealer then how to claim the tax which he paid under reverse tax.