eBay best offer and seller coupon

No. Don't think so. I made an offer to a seller once thinking I can use a discount code as well, didn't work

No, I got stung by a coupon for 15% and a best offer on a £180 item, after the seller accepted the best offer I made that was 5% under asking price the coupon disappeared which meant I was 10% worse off.
The seller was a big shop so I explained to them my loss and asked for the sale to be cancelled which they agreed to.

No. I too have made the same error and in the end lost out on about 8% when i did it.