Select sole proprietorship and register as self employed with HMRC to get your UTR, you’ll need this for when Amazon do their checks (when you hit a certain level of revenue).
You’ll be surprised how quick you might reach 25k in sales for the year and then start looking at tax implications. Bearing in mind in a dream world you might make an extra 5k profit on that 25k
You might get away with it for a short time, or not at all, but if you are buying to sell to make a profit you need to be registered as such with HMRC. As soon as your turnover (not profit) is 1K HMRC requires you to be registered. Amazon require you to be registered from day one.
Yes that is exactly what you would do, but you need to be registered in advance as Amazon will request your UTR number (and household bills etc to verify you)