In my experience the paper label inside the tongue will have a number code on it that can be researched on google. Its a model number or style number I think, although I don't believe it says that on the label.
Yes, the sneaker market is huge for folks who know what they are doing. I e gotten lucky and sold more than a pair or two for over $100.00. Met a guy on eBay who does at least six figures a year just on eBay sales. He also sells on Instagram and Facebook.