Can I trade my iPhone 11 for an iPhone 11 Pro Max?

No. Since the iPhone 11 is one of the new models (not even 4 months old at this point) it is not eligible for trade-in.

If you bought it less that 14 days ago from Apple, you can return it for a full refund.

Otherwise, if you want to upgrade to an 11 Pro Max, sell the iPhone 11, and use the money to buy the 11 Pro Max.

You will have to ask at the Apple Store, phone in hand. If the 'old' phone was purchased less than 14 days ago, it can be returned for full refund by the original buyer.

We're just regular users like you and have no say in what Apple will do.

Trade info...Apple Trade In - Apple

No. Apple does not accept the iPhone 11 in trade at this time.

eBay, CraigsList, some local users club, etc.

Undoubtedly not until after the next generation of iPhones is released.