The paper aims to highlight the lack of usage of knowledge-based expert systems in purchasing decisions in the context of hybrid corporate reality. We use the transdisciplinary approach in our work, which is essential to examine the problem that occurs in the reality. While reviewing publications containing the keywords “Cognitive bias” and “Supplier selection”, we focused on the methods used. The examined methods in the pooled papers are mainly based on arithmetic and rank the possibilities without considering the available expert knowledge then and there. Afterwards, we propose a solution beyond analyzing the data measured in the past; in addition, the decision-maker, their mental model, and their knowledge is considered. We assume that the effects of cognitive biases are more readily identifiable when using expert systems in considering the decision-maker’s opinions in connection with the actually applied rules in making decisions. In addition to seemingly objective solutions, in our experiment, we propose that by using past cases with known results, complex rules, which are based on the expert’s knowledge, can be simplified without changing the results of decisions in purchasing.