I’ve had great luck with On Course in Wellington. I’ve sold a bunch of things with them and they’re great.
Online-wise, I’ve used Pik & Louie and Redwood. Highline also has a great rep.
The Maryland (MD) Tack Exchange in St Augustine, FL gets a big thumbs-down from me. I had an older Butet for sale at a local consignment store. They came in and told the owners a fake story, and asked to negotiate me down off of an inexpensive price by lying and saying it was a for child on a budget. So I sold it for $200 less thinking I was helping one of our next gen’s riders get a leg up. They bought it and it magically popped up on their Ebay for $1000 more than I had it asked for it. Not only did it take money out of my pocket, but it also took money out of the local consignment store, who felt so bad about getting scammed that they took less commission on it to make up for it. Totally shady and I’ve never forgotten it. In fact, I love telling the story in hopes that it takes away from their future sales. What goes around, comes around.