It kind of depends, in general reselling is very competitive. You have to find a good niche i guess.
What i like is flip higher ticket items like motorcycles, scooters, small cars because it's more worth my time. Meeting with buyers/sellers usually doesn't take much more time for higher price items than low ticket items. If you let's say sell something worth 50 and profit margin is 20% that's only 10 profit. While a motorcycle of 1000-2000 price range can bring in few 100 at least.
Niches are also a thing. For example i'm a swiss army knife collector and sometimes on ebay limited edition knives go for not that much when people are in a hurry to sell. Everything with some collectors demand can be worth wile but it can take some time to flip.
A good forum where reselling opportunities in North America are discussed is https://www.reddit.com/r/Flipping/
There may be a lot of competition, but few people reach basic competence in all relevant aspects:
- Estimating value and demand
- Selling on the most suitable marketplace
- Writing a detailed and attractive description
- Taking decent pictures
I agree that you need to specialize in a certain area in order to recognize good deals. I'm not sure about selling on niche markets. It depends. Prices might be higher, but that won't help you if there are a hundred times more buyers on a general e-commerce site.
My experience is in selling used electronics and other stuff from myself and a friend. Best sale: two computer/TV monitors that were still wider than many new models, but with a slower refresh rate. Both gone within a week for a good price. The online marketplace's algorithm underestimated the value, so I was able to advertise them for cheap.
Most difficult sale: an Italian motorcycle helmet, never used. It didn't fit the first two guys who tried. Clothing sizes in general are annoying.
I have some used books to sell, but I might just give them away. It's not worth the effort, and you need to wait ages until someone's interested in that specific book. I think most people who sell second-hand books are book lovers who do it as a hobby while barely making a profit.