Stella McCartney Bags

The RealReal is a community of about 40 million members, the largest platform for authenticated luxury resale in the United States. Founded in San Francisco, California in 2011.
The currently have 17 stores in: California, Connecticut, Florida, Illinois, New Jersey, New York and Texas.

FWRD Renew is the pre-owned division of FORWARD/REVOLVE, Forward (now known as FWRD) is a firm from Los Angeles founded in 2011 by the well known fashion firm REVOLVE and originally managed by Elyse Walker, known for her high end celebrity boutiques.
Their FWRD Renew division offers a great selection of pre-owned high-end designer clothing and accessories.

This firm is based in Dubai, founded in 2012; and gives you access to a huge market of ultra luxury items from a very wealthy region of the world. For that region owning Stella McCartney products and Stella McCartney Bags is very common, some of the people there wear such items daily.
The Luxury Closet has a mix of items they stock in their store and are selling directly as well as items they are selling for regular people or items other commercial sellers list on their site.

Rebag, was founded in New York City in 2014. They offer a mix of their own inventory and inventory on physical consignment (they physically take in the item to verify its authenticity and condition). They have boutiques in California, Florida, and New York. They have one of the largest selection of pre-owned Stella McCartney bags.

Based in New York. They were originally an art auction house and now they are one of the important online sales venues of ultra luxury items.
They have great pictures and depending on the seller they will have enough information about the items, they provide an easy way to contact the seller and you can even make an offer directly on the listing.
As far as pricing, like many other sales venues, the price will reflect an increase due to the commission they charge to the seller. Still there is a constant number of good opportunities to find.