I can't help with finding collectors, but when you do, just remember you CANNOT ship ammo by mail (USPS). It can be shipped by UPS ground or FedEx ground (no air) by packing it well in a cardboard carton with no printed advertising type markings on the carton. Label the cartons, add a "ORM-D SMALL ARMS AMMUNITION" label or simply mark that with a black Sharpie marker and draw a square around the lettering.
Then take the package to a UPS hub or FedEx hub and ship it. The only cost is standard ground charges. It is not hazardous and there's no hazmat charge. The shipping point MUST BE a hub, not a storefront retail store like Kinko's or The UPS Store, as they will not accept ammo for shipment.