Retail pays in two ways: the margin, and the result. A client using the right product at home keeps the work looking good for longer, which shows up in your reviews and in how often they come back. The second is usually worth more than the first.
The cost is that stock is cash sitting on a shelf. Money in bottles is money not in your account, and slow-moving stock is the most common way a small studio ties up its working capital. That is the whole risk, and it is why the answer is start narrow.
Stock what you already use. Three or four products you genuinely reach for during appointments, in the sizes people actually buy. You can recommend those honestly, which is the only kind of selling that works at close range, and the client has already seen them used on their own hair.
Sell by demonstrating rather than by pitching. Say what you are using and why while you are using it, and let them ask. A professional who explains is trusted; a professional who closes is a shop.
Watch what sells for a season before widening the range, and be honest when nothing does. Some books do not buy retail, and carrying stock for them is a slow loss with no upside. Tax and takings are worth keeping separate from services in your records either way — products and services are often treated differently.