I think you have done it the wrong way round. First you need to do your research, read seller university forwards and backwards, read all the regulations regarding distance selling, product standards, your obligations as a business/sole trader to register correctly (I presume you have already done that), find products from the correct sources, that Amazon will be able to see you are authorised to sell on their platform, then register, once you have a solid (ish) business plan. You could watch the TubeYou and Faceplant videos, then don't do what they tell you (everyone thinks they are experts on these, but just copy other's bad advice)

However other sellers products and sources are based on their extensive time and research, and part of what makes their business work, so it's unlikely any of us are going to give that information to would be competitors, sorry.

It is often easier to sell a product you have some knowledge of, at least while you get going. If you've previously worked with a product or use it regularly for a hobby/dayjob you're much more likely to know any legislation around it's sale. No one is going to point you in the direction of their own best sellers as that would be business madness, you'll need to work out what you can sell and make a margin acceptable to you. You might go high volume low margin or low volume high margin, the latter is easier if you can find your niche.

Read the forums diligently, search for things such as 'account deactivation' and you'll soon find recurring themes which tells you what not to do. Don't watch TikTok/YouTube video's of gurus telling you how to make retail arbitrage work etc, those people make their money from video views rather than trading on Amazon and are sometimes misleading, following their advice is often the quickest route to account deactivation.

Know the difference between a wholesaler and a retailer. A simple litmus tests is can you buy the products direct from their website, if you can they're generally a retailer. A wholesaler will display the brands they represent with but will conduct business by email/phone and once an order is agreed/accepted provide a pro forma invoice for payment and final invoice at/after delivery. In time you maybe given a credit line with them forgoing the need for pro forma payment. Before completing any purchase ask the wholesaler if there any restriction put in place by the brand regarding sales on Amazon. Where possible also make sure you'll be able to get ungated to sell the product on Amazon before purchase (not always possible until you have a final invoice).

You may or may not be able to ship direct from the manufacturer/wholesaler to FBA. Sometimes other actions are needed such as liquids put in polybags or barcodes needing replacing. Sending something in and then having to recall it to make an adjustment is a very costly error in time and money so get it right. Amazon expect you to know what you're doing from seller university so most interactions you have with Amazon are AI replies, Seller Support staff can't know everything about every product (or every Amazon policy) so can give incorrect advice.

Do you expect to sell more than £245 worth of items a day on average? If so you also need to prepare for VAT registration and understand how that will affect your business model (it can be positive as well as negative). Know your costs inside out and allow a margin for returns/return postage, then whatever margin you thought was acceptable for returns quadruple it and run your numbers again.

Don't put all your eggs in one basket, just as you think you have its sussed and order a container or two of your best selling product a competitor will crop up who is happy working to a lower margin, or worse Amazon start selling the product themselves. Selling on Amazon can be productive but you need to understand the nature of the beast to avoid costly mistakes.