Hello JK and, on behalf myself, welcome to the GMIC website. I hope you visit often, post photos, questions and your insights.
I don't think your question is stupid. In fact my answer will likely not be universally agreed upon by others. But here is my attempt.
No, a dealer or website doesn't offer much in proof of authenticity other than a 100% money back guarantee. The good dealers will provide a money back guarantee and therefore it is a good idea to keep good records and the receipt of purchase. A dealer in good standing, and with a long history will also be an expert in the field and will generally only offer legitimate items for sale. It is only if the collector loses confidence in the piece or it because recognized as false that the money back option is needed. One dealer would provide a documents with the description of the item, a photo along with a date and purchase amount as a form of a guarantee. These became so recognized as a seal of authenticity that forged items would have forged Detlev documents!
The best hope (for all of us) is learn everything you can about a legitimate items construction, known dimensions, maker marks, etc as well as the traits of known fakes. Then you can purchase with greater confidence from good dealers that will know the items they sell and offer refunds. Ask stupid questions. Dealers love to discuss the items and everything they can about them. It's is best way to get the most enjoyment out of this hobby.
Regards,
Claudius