Edit: SOLD!
I “lost” my EasyCard with monthly pass and bought a new one only for me to find my old one, now I have an extra, brand-new TPASS card with the $1200 monthly amount topped up on it.
I’ll include all the original receipts, brochure, documents that go with it and can mail it to your address or a convenience store; I can also meet up in Taoyuan, or Neihu during lunch hours M-F.
Payment would be cash, bank transfer, or PayPal.
If you stop at one of the handy MRT customer service booths I believe you can explain what happened and get credit for it, which is probably much easier than the hassle of selling.
I actually just tried that at noon and they said they can only refund a usable balance but not the monthly pass $1,200 bit. I tried telling her what happened and got the 沒辦法 line. 
Sold!
If you report the card lost / stolen / damaged, EasyCard should be able to refund it. I had that happen with one of my previous monthly Taipei passes (before the 1200 passes), it took a few weeks but they gave a pro-rated refund of remaining days. You can ask any of the MRT guard stations and say you want to fill in a lost/damaged card form, they will write in that day for pro-rated purposes.
