Let's assume you write an e-mail to a bike seller and issue a question:
Sehr geehrter Herr Müller,
ich bin der Käufer des Fahrrades Nr. 123. Ich hätte folgende Frage an Sie: [...]
You are the only buyer, but you want to leave it unspecified whether the buyer is unique or whether a community of a few people has bought the bike. How to do it? Do you need an article before "Käufer" or can you leave it out?