Is the seller on Amazon Kindle sinful if the customer purchases his product through usury or an illicit method, or with ill-gotten money, knowing that the product itself is permissible?

There is no sin upon the seller if the buyer's money is unlawful, because the seller is not obligated to inquire about the source of the money. The default assumption is that the money in a person's possession is lawful until proven otherwise. Muslims have dealt with Jews and polytheists despite knowing that they engaged in usury and did not avoid forbidden things. There is no disagreement among the imams regarding the permissibility of dealing with someone who possesses money, the origin of which is not known to be usurped or unlawfully acquired.