It's the same batch of sellers, selling across most all of those sites. Or to the extent that there are sellers who are, say, on Ebay but not Amazon, or vice versa, they have bots tracking the prices across all the sites. In either case, I find that you just don't get much price variance across book selling sites these days. I mean, sure, there are occasional exceptions, but by and large, prices tend to converge to a common value, or at least to a small range around that.