Wanted: Mid-2012 or Early-2013 Macbook Pro Retina 15"

Interested in a Mid-2012 or Early-2013 Macbook Pro Retina 15", preferably with 16GB RAM, 512GB SSD, and Nvidia discrete graphics. Prefer to pay less for one in worse condition (poor battery, cosmetic defects, etc.) as long as it still functions. Interested in paying $520, but willing to negotiate, depending on condition.