I have sold on Amazon but not eBay, and the method I was using was purely retail arbitrage. There is money to be made doing retail arbitrage but it can be more time consuming and frustrating (i.e. when you do find an item that is a good seller but you cannot find more inventory in your area). That is what then often leads people to pursuing wholesale so you can buy in larger quantities and at cheaper prices. I never made it to this point of trying wholesale as I started to lose steam on the time it was taking to do retail arbitrage given other commitments. Lastly, you have private label products where you put your own brand on something and sell it. Some people are doing well with this, and some that teach the courses on how to do it are doing really well. =)
All in all, there is money to be made but it can take a lot of time--and you can lose money too as you learn. There is a guy named Jordan Malik who puts out a lot of good free information, and I still follow his emails. He has done eBay and Amazon among others. Another person is Jim Cockrum, he has a Facebook group called My Silent Team and he has a bunch of courses available on these topics. There are countless others but these two guys are two that in my opinion always go above and beyond to deliver value to their audience.