apple watch was rejected for trade in

I recently purchased an Apple Watch series 6 with the agreement of sending in my current Apple Watch series 3 with a $70 trade in value. (ie Apple would give me $70 back after receiving my series 3 watch) I sent my series 3 watch back in the Apple provided box (via UPS). A few days ago, I received an email saying my watch was not in the condition that I specified and the trade in offer was no longer valid. No explanation was given.

My watch was in impeccable condition, not even a scratch on it, and was most certainly in perfect condition. Does anyone know how I should move forward? Should I visit the local Apple store? Should I just sell the series 3 on my own?