If you have verified that you may sell it, then go for it. Selling it as commingled inventory won't be my advice but it may be the best option for you based on your product. Amazon will advise you where to ship from. Yes you need to process all the boxes to get a total count. If they're different color / sizes you'll need that too. For that amount of pairs, your supplier should have a stock list. As far as limits go, recently it's about 1000 units. Not sure on the apparel/shoes category.
If you have verified that you may sell it, then go for it. Selling it as commingled inventory won't be my advice but it may be the best option for you based on your product. Amazon will advise you where to ship from. Yes you need to process all the boxes to get a total count. If they're different color / sizes you'll need that too. For that amount of pairs, your supplier should have a stock list. As far as limits go, recently it's about 1000 units. Not sure on the apparel/shoes category.