Matchmaking in e-marketplaces is challenging partially because of the multifaceted nature of participants' expectations and interests which makes the modelling of participants' profiles difficult. We formalize a profile as a set of constraints, each consisting of an attribute, a set of values, the constraint flexibility and its priority. We analysed several Knowledge Representation models (KRM) used for developing matchmaking systems. We propose a new KRM for Web-based matchmaking systems that can represent these constraints. We describe the development of a matchmaking system that implements the proposed KRM, exemplifying its features and evaluating its performance.