So maybe instead of "demanding documentation" and feeling threatened you should use this opportunity to improve and gather user feedback in a friendlier way.
Just my 2 bitcents, take care
I think what the Bidding Pond user is mad about is less the claims that the site's UI isn't entirely pleasant, but Charlie's claim that many users were unsatisfied with the site's service. There's a big difference between the two and I can see why he/she/it would definitely want to request documentation, both to get to the bottom of any potential problems and to protect the site from unfounded but potentially damaging rumors.
Stating that your user experience with someone's site was less than idea is totally fine. Stating that many people were unsatisfied with and have complained about a site's service, without the ability to back up this assertion, isn't.