Got a ‘winning’ product, now what?

So I’ve narrowed down a product niche I can see a gap in the market for and now I have a few questions on where to go next.

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These products are sourced from alixpress, do Amazon accept a manufactures invoice from them? So I’m aware I should create a trademark brand, is it as easy as just registering the trademark or do I need to set up a LTD company etc (UK)

Thank you for any help.

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