No competitor is going to give you the benefit of their experience, that's hard earned and often an expensive learning curve they've had to go through so won't be giving up that info for free. Suppliers will also be well guarded secrets.

That said, some of the information you're asking for is freely available on Amazon anyway, to see the best seller in a category, look at any product page in the category, scroll to 'Product Information', open 'Item Details' and you'll usually find the 'Best Seller Raking' which you can open and see the best sellers (often top 100) for free. Beyond that there's software (usually not free!) available that'll offer more in depth analysis.

Getting ungated to sell those items is another matter entirely!

Authenticity shouldn't be a problem if you're buying from genuine suppliers in the country you aim to sell the products in and have done your due diligence of establishing supply chain back to the brand before completing the purchase. The formulation of beauty products can vary from market to market so your supply will need to be market specific. There is usually one or two people a week coming to the forums having lost their Amazon account due to buying from other marketplaces such as Quogita rather than from genuine wholesale suppliers so don't fall into that trap.