Can I still trade in my new iPhone after purchase?

You can trade it in and you will get a gift card with its value.

click here ➜ Apple Trade In - Apple

It won't be applied to your purchase charge to reduce the total paid if that is what you are thinking.

How did you pay for the new phone? If you happened to use an Apple credit card to purchase your new phone, you can apply the gift card you will receive toward your bill, or so I was told by the salesman.

This is incorrect.

You can only apply the trade in value to the purchase if the trade in and purchase are done at the same time.

If the purchase has already been done, then, while you can trade in another iPhone separately, you will not be able to apply it to the charges on your credit card as all you are getting it an Apple Gift card that can only be redeemed onto an Apple Account, not a credit card bill.

If what you mean is that you want to trade in your old iPhone, see Phil's answer.

If you are saying that you want to trade the new iPhone that you just purchased, you may not be able to trade it in. Apple typically does not accept trade-ins the latest iPhones (in this case, the 18 Pro, 18 Pro Max, and Duo). If you purchased the new phone directly from Apple, there might be 14-day return period during which you could return the new phone (so you could then apply the sales price towards another phone).

How did you pay for the new phone? If you happened to use an Apple credit card to purchase your new phone, you can apply the gift card you will receive toward your bill, or so I was told by the salesman.