Are you interested in buying an encyclopedia?

Literal

Encyclopedia [object-を] buying [want-たい] [quotative-という] interest [topic-は] exist [question-ますか].

百科事典を買いたいという御関心はありますか is very formal/polite — 御関心 uses the honorific prefix ご with 関心 ('interest'). という links the desire to buy with the noun 関心. Sounds like a salesperson's pitch.