I guess you really don't.....but if the sellers wanted to set a minimum price on an item, they would put the reserve price at whatever amount they wanted to get out of it. If they don't get the minimum amount, they can relist it until they do.
They can still register multiplie accounts which would show the same IP address or at least the same range if they aren't using a static IP. They could be using another computer. Or they could have someone they know big on it. If someone wants to they can. It's not that hard. If you suspect something then talk to eBay.
