Purchased a gaming desk. As per the listing the item’s packing may be damaged but item is intact and comes without assembly instructions.

Received the item in a mangled packing and main piece broken. Contacted the seller and received 20 % discount. Proceeded to assemble the desk and towards the end came to know there was a missing piece too(since no assembly instructions, has no idea about the parts). Tried to to contact the seller to resolve again but no response. No help from eBay as I already accepted the 20% discount for the broken part. 

Can someone suggest how I can resolve this since eBay is not helping either?