Declare the correct value on the Customs form?

what someone else said. You’re returning used goods. There should be no charge.On the other hand, what customs inspector is going to question a value of $50 for one LP? No LP ever made is worth $1200. They might be more inclined to question such a huge value. Then there’s your own self interest; you want your money back.

Ouch. I hope you can afford the loss on this, but it doesn't sound good. 

You don't even want to know my thoughts on this...

Lots of holier-than-thou people assuming that someone is cheating here and ignoring the fact that noting is being imported so no duty is due.

My holier-than-thou assumption is you're out 1200 bucks.