The seller of these items won’t see what we see, they would see a fair price to be a little % off new, but the issue is Anker do periodic discounting of almost all products, we find them and share them, so most here would see 20% off as basically list price for new, most here would wait for a 20% discount before buying new items, and want additional 20% minimum off new for used, so would be seeking 20%+20% 40% minimum off. The person selling is likely unaware the item is being sold often new for 20% off and so is basically asking for used what you can pay new and see 40% off as unfairly low.
I expect the seller would be rejecting 40% off offers thinking totally unreasonable.
The seller would just have to learn. Best place to learn is Ebay, not this email game. Put a reserve on item and let bidders decide the selling price, ebay is setup to solve exactly this problem.