If I send a buyer an offer for 20% off the listing price, but I also have a 20% off storewide coupon active, will the buyer get an additional 20% off the offer price, or is the offer price the final price they pay?
No, they don't. If you send an offer or the buyer sends you an offer and you accept, the coupon will not work.
"If several discounts are running concurrently on an item, your buyers will get the best discount. If an item has a Best Offer or Offer to buyers, the offer takes priority, and the coupon cannot be applied."
