I am confused. I am not a finance or tax student, and I am finding it quite hard to wrap my head around this concept. How does a shop that sells refurbished phones work?

How do they enter into the books the phones they buy from unregistered individuals, sell those phones under GST, and issue invoices that follow the law while also protecting business secrets?

As far as I understand, for refurbished electronics, taxation is calculated on the margin (purchase price – cost price + refurbishing cost, etc.).

* **My doubt is:** If I show this invoice, it will reveal my margins, which is not good for any business. Customers may reverse-calculate the profit margin and start negotiating the price after knowing the margin, right? I asked Gemini 3 Pro, and it said I can show only the final price (inclusive of all taxes), but when I asked another AI powered by Gemini, it said that showing the tax amount is mandatory for transparency.
* Do I charge GST from the customers, or do I pay it myself?
* If I bought a phone for ₹10,000 and I am selling it for ₹14,000 (inclusive of tax), what amount should I ask the customer to pay — ₹14,000 or ₹14,720 (₹4,000 + 18%)?

Any additional information is welcomed

Additional/Optional

Is there any software (like pos/erp) that can do this for me ? Like record purchase and sales in GST complient manner and also generate bill in appropriate format ?

Actual query related to GST (Margin Scheme). I’ve been trying to solve this mystery for the past 15 days.