What's good product or niche for beginner with guarantee of sales?
I cannot advise you on a specific niche or product, but when choosing a niche for a beginner, I would give the following tips:
Do not choose a niche where there is high competition for the main keyword (if your search for the main keyword shows you more than 200,000 products on Etsy). With such competition, it will be very difficult for you. My advice is to look for niches with the issuance of no more than 100,000. For example, the jewelry niche is very interesting, there are a lot of sales, but there is crazy competition, and in order to start selling well in it, you need good financial investments.
Also, I would not advise you to choose everything related to clothing / shoes as your first niche. There are a lot of returns in these niches due to the fact that people cannot choose the right size for themselves.
When choosing a niche and product, it is important to understand - do you plan to make the product yourself or do you want to do dropshipping?
If you want to engage in dropshipping, then you can choose absolutely any products and niches, make a store for each group of products, and that's it.
If you are planning to make the product yourself, then you need to understand what you can do? For example, I would love to sell metal wall decor, but I cannot make this product myself, and finding a good artist is a problem.
If you tell me what you could do yourself, I could give you more specific advice on what you could sell.
If you are interested in dropshipping, then choose whatever you want.