I buy lots of coins on
ebay. I buy only those that have good pictures taken with a good camera. I am getting better at telling cleaned coins. I look for parallel hairlines on the raised surfaces and too shiny surfaces, among other things. Once I get them I look at them with a 40 power microscope for altered surfaces.
At first I was getting 2 cleaned coins for every ten I bought, but now it happens very seldom. I have learned which dealers I can trust. I always buy raw and sent them to NGC for grading, then resell them.
I average a 20$ profit.