I know people will hate me for saying this, but start high. As far as I know you're the first to sell the new bumper, and I think some people might really want this.
But you do need to list a price, and a picture helps a lot for a sale.
To buyers: I'm not sure we have the electrical connectors on our pre 2013 models' bumpers. You might have to buy one and install it.
EDIT: Actually if you (the seller) could find a way to include it with the bumper that would also help you sell