But sometimes the product description doesn't mention it being in new or used condition. It just says collectible. I think they mention collectible just to hide the fact that it's already been used. It's only what I think.
Amazon has very strict guidelines regarding the listing of items, IIRC only Books, Music, Film, and Toys can be listed as collectable, for example Books must be First print, limited print, or signed by author to be considered collectable.
The toys category is more vague as it merely states that the item "Must be unique" and that the seller is able to list why the item is collectable.
So to answer your question, collectable items on amazon could be new, or used, but as they are collectable items you should always be able to find information about them before you purchase, either through the listing or contacting the seller.
Generally, they're used. Like others have suggested, check the description and ask the seller, so that if the item is not in the condition you want, you have a trail of e-mails as proof that you didn't receive the item as described.