hello if my screen have a small scratch and still available apple warranty 1 year so can i replaced it for free?

A scratch would be considered accidental damage and is not covered under warranty.

So Applecare won't cover a small scratch to touchscreen (they say cosmetic) but will cosmetic damage affect the trade-in value? Apple offer is $450 max to $110 with damage to touchscreen. I'd hate to send it in and then only get 110. Thanks

If it has a scratch, that will make a difference in value - nothing you can do about that.