Everything you need to judge whether a category is worth entering is on its own page, and almost nobody reads it that way.
Sort by best selling and look at the gap
If the top item has many times the sales of the fifth, the category has a winner and entering below it is hard. If sales are spread evenly, buyers are choosing on fit rather than reputation, and a newcomer can compete on being the better fit.
Read the price spread
A tight cluster means the category has settled on what it is worth, and undercutting will not win you much. A wide spread means buyers are unsure, and there is room to position.
Check the update dates
Sort by recently updated. If the leaders are absent from that list, the category is being defended by nobody, and that is the single strongest signal you will find.
Count what is missing
Scan the titles. If every item is a general version of the same thing, the niche versions are unbuilt. If they are all niche, the buyers wanting one tool for everything are unserved. Either gap is a product.
Look at the item counts on subcategories
A subcategory with real traffic and few items is the clearest opportunity a marketplace offers, and it is visible from the outside before you write anything.
Related: choosing what not to build.
