This paper attempts to model the marketing and sales process of high-tech start-up companies. It correlates the notions of the marketing mix (the 4Ps) with the probabilities of success of various phases in the sales process. By applying the formalism of reliability block diagrams used in engineering, the total probability of the sales process can be expressed as a function of the probabilities of all the subprocesses. A particular insight resulting from this work is that the total probability of the customer buying decision corresponds to the relative market share of the company. Discussing the allocation of investment in the specific marketing elements of this model will improve understanding of how management can achieve a rapid increase in sales and so accelerate the development of a high-tech start-up company.