Three things, and the third one is the reason the seller matters as much as the invoice.
First, a proper tax invoice. It must carry the seller's GSTIN, your business's GSTIN and registered name, the product description and quantities, the taxable value and the GST amount shown separately. A retail bill with only a total, or an invoice in an individual's name, does not qualify. Ask for the tax invoice on the company's GSTIN at order time; converting it later means a credit note and a fresh bill, and that is avoidable paperwork.
Second, the purchase must actually be for business use and booked in your accounts accordingly. Conference room audio, showroom sound, studio monitors, a demonstration theatre, a hospitality space: these are the normal cases. A home system in a director's living room is not, and dressing it up as one is a conversation you do not want with an assessing officer.
Third, the seller must actually deposit the tax and file it, so the credit reflects in your GSTR-2B. This is the step buyers forget because it happens on the seller's side. Your credit is only as real as the seller's compliance. An invoice from a registered, authorised, tax-depositing dealer appears in your 2B where it should. An invoice from a dubious seller may never show up, and when that happens the credit is denied to you, not to them, and you are chasing someone who has already stopped answering the phone.
That third point is why "cheapest quote" and "claimable input credit" are sometimes opposite goals. A seller shaving the price by not being fully in the tax system is handing you an invoice your CA cannot use.
The mechanism and terminology (2B, filing timelines) are standard practice as I write this; check current rules at the time of purchase, and let your CA confirm eligibility on your specific purchase, including any blocked-credit categories.
Our part is simple: give us your GSTIN at order time and the invoice is raised on it, itemised, from a compliant channel. The rest is between you and your accountant, and we are happy to send whatever documents they ask for.
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