It's a rear infill panel that was an accessory that people used to fit between the rear lights to fill in where the number plate usually sits. It should have a mounting kit with it in the box but your first picture isn't very clear.
Most of these panels just said "Capri" on them but as yours is 2.8i injection specific it will limit it's market. It should have a bit of value; here's a Capri one on ebay https://www.ebay.co.uk/itm/Ford-Capri-M ... SweExaFceo
Truthfully I think it will not sell for that much. Ebay may be your best route for selling it unless someone on here wants it.