http://forums.backpack.tf/index.php?/topic/36705-guide-for-non-unusual-suggestions/

Classifieds listings. We are now accepting classifieds listings as primary proof provided that they meet certain criteria. First and foremost, you MUST provide a screenshot of classifieds in your suggestion. (1) Sellers: There must be at least 10 different sellers at a given price OR, for rare items, the number of sellers at a given price must be greater than the total number of trades up on outpost or other sites.

I think number of sellers being the same as outpost trades is probably enough, as it shows it's still a common point in a small market.