Is it permissible to take a commission on the sale of a halal product, and then take another commission when the buyer sells the same product, knowing that the marketing principle involves explaining the product and then convincing the customer of the commission they will receive if they sell it? And do I bear the sin of the sale if it is unlawful?

The basic rule is that brokering and mediation between a buyer and a seller to facilitate a sale are permissible, and it is permissible to take a commission from the original owner of the goods, as long as it does not involve any religiously prohibited element. However, taking a commission every time the buyer sells through you has no basis and falls under consuming people's wealth through falsehood, which is known as forbidden multi-level marketing.