Indian business income, and the TDS the marketplace gets wrong
Selling goods into India through a marketplace means you have Indian business income, because the sales arise through a business connection in India under Section 9, taxed on your profit and subject to your country's treaty and whether you have a permanent establishment here.
The withholding is where the marketplaces trip up. Everyone points to Section 194-O, under which a marketplace deducts a small TDS, now 0.1%, on a seller's sales. But that section applies only to an e-commerce participant who is resident in India; a non-resident seller is outside it entirely. So the marketplace should not be deducting 194-O on you, and TDS on payments to you, as a non-resident, falls under Section 195 instead. In practice the platforms' systems default to 194-O for everyone, so an NRI seller often has the wrong TDS applied, which then has to be untangled and reconciled. Getting the right section on from the start avoids that.
GST you cannot avoid, and it is separate
GST is a separate tax and, for a marketplace seller of goods, unavoidable. Normally a small business only registers for GST once its turnover crosses a threshold, but a person selling goods through an e-commerce operator must register regardless of turnover, there is no small-seller exemption for them. A non-resident selling into India registers as a non-resident taxable person, which itself carries no threshold. So GST registration is a given from your first sale.
On top of that, the marketplace collects a small amount of GST at source on your sales and pays it to the government against your registration, at a rate that was reduced to 0.5% in total. That is a GST mechanism, entirely separate from the income-tax TDS discussed above, so on a single sale you can have both an income-tax withholding and a GST collection running, two different taxes to two different systems. A practising CA gets your income-tax TDS onto the right section, handles the mandatory GST registration through a representative in India, and keeps the two taxes properly separated and reconciled.