To help keep it simple, ask 2 questions when you see something that isn't listed (after determining that it isn't a "trading card proper" because all trading cards are eligible):
1 - Was it included in a trading card product (insert, bonus, etc.)?
2 - Was it manufactured by an established trading card company?
If the answer is yes to either of those, it's eligible for inclusion here.
If the answer is no to both of those, it doesn't automatically exclude it, however, that would be a good time to ask in the forums (or one of the members who has a really firm grasp on the rules and guidelines) to see if it is eligible.
-- Dan --
Note: Please see my profile for more info regarding trading (section updated 10/25/2023). I have added a large portion of my inventory to the site, and currently have trading turned off (details are in my profile). I have (finally) unearthed my 2 very large boxes of Star Company minor league sets and they are available (email for details).