If you copied and pasted the serial number, and the Apple Trade-In site still didn't accept the serial number as being valid, then you should follow the suggestion that TheLittles made to contact Apple Support.
I was just trying to see if the problem was with the transcription of the serial number, rather than with how the Apple Web site viewed the actual serial number itself. I have purchased a number of Blu-Ray and DVD movies which come with long alphanumeric codes for redeeming a Digital Copy - and with the printing on some of the inserts, you can tear your hair out trying to find out why the redemption "won't work" until you stumble upon a substitution that could have been avoided if they had used a larger font, or avoided ambiguous characters.
Here, the Mac gives you the ability to completely avoid such transcription errors. So now, when you contact Apple Support, you can tell them that you copied and pasted the serial number - and you will not waste time trying to chase down a possible transcription error that doesn't exist.
And when they ask for the troublesome serial number, you can copy and paste it again to ensure that you're giving them the right serial number and are not accidentally leading them on a wild goose chase.
I'm sorry that this hasn't solved the problem, but at least it has narrowed down the list of possible causes.