There are many methods to assess the usability of interactive software systems. The Heuristic evaluation is one of the methods that allow finding many usability issues, based on a set of usability principles (heuristics). However, sets of generic usability heuristics may miss domain - related usability aspects. Particularly, transactional web sites have specific features which could potentially not be considered when using generic heuristics. We developed a set of usability heuristics for transactional websites, using the methodology proposed by Rusu et al. (2011). The set of heuristics was designed, validated and refined during three iterations; experimental validation indicates that it is an effective and efficient evaluation instrument. The methodology facilitated the heuristics' design and specification. However, certain deficiencies were detected when applying it, so further improvements are necessary.