No win situation for you selling branded shoes from China unfortunately, and no one will be able to supply you with legit proof of authenticity as it just doesn't exist in China. It doesn't matter what it is, shoes, phones or whatever, no chance at all of getting authentic branded items. That's why eBay have jumped on your account.
In regards where you can find them, I guess it comes down to what brands, but something like Nike is zero chance. Big brands like this market through their own channels so they can maintain control over their brand. The last thing they want is Nike shoes turning up on eBay at half the price you can buy them for in the stores.
That's not good for their business lol
My best advice, check out surplus suppliers. No guarantee you will find them and even if you do, you may be limited in how you are allowed to sell them.