Back in the sixties, a company called Gorlier devised a marketing technique destined to become famous and much imitated.
It was based on the idea of giving their encyclopaedias away free, instead of selling them. The salesman would tell you the encyclopaedia was being offered to you free-of-charge, in return for your endorsement of the product, to be used in a future advertising campaign. Great! “I’ll take them”, said everyone, but – and, of course, there had to be a ‘but’!