What is your goal? To build a business so you can quit your job? Or just make a little side money? That will really inform your strategy more than anything. How much do you need to make? If you are funemployed now, building a flipping business is not something you can launch right away unless you have significant capital and know where you will source your products (or you have a very low cost of living)

Personally I would recommend ramping up slowly as a side hustle while using another main gig to pay your bills.

Markets and trends fluctuate a lot and sometimes stuff that was selling a lot just stops or other sellers start listing the same items because they are following trends. There are pros/cons to every marketplace (facebook, ebay, poshmark, etc). There are different strategies for each of those, different pricing models and inventory models (how many units you need to have on hand, etc).