What is the ruling on a company selling da'wah (Islamic propagation) tapes for money, even though its activity is purely commercial?

It is forbidden to participate in pyramid marketing, even if the company sells real goods; because the intention is not the goods but the marketing, which is based on gharar (excessive uncertainty/deception) and consuming people's wealth unjustly. The participant pays money to get a chance to convince others to buy, and to get a commission. The purpose of buying becomes the commissions, and the product is secondary. This is forbidden by Islamic law due to the excessive gharar it involves, as the participant may not succeed in convincing others and thus lose their money. Every layer in the marketing pyramid will be a loser, unless there is a layer below it that bears the loss.