If I spoke to such a person, and I can't think of why I would, I would say:

'Thanks for making my prices look great. Also for making my photos, descriptions, and the work I do on the bikes I sell look awesome. By comparison to you I look like the freaking genius of bike history and mechanics, and like I'm the most trustworthy guy in the world. But words alone can't express how much you have helped me, it requires something more tangible. So here's my warmest handshake.'