Hi energizedit,
I would say Sedo, it seem Sedo have a better platform. Flippa is great if you going for fast auction however you are putting up against a ton of other domain that is also going for auction. Can be hard to sell sometimes.
Have you consider other platforms. You can actually sell your domain to at the same time on multiple platform as long as you read their term and nothing mention regarding to exclusive right to sell your domain. Exclusive right mostly happen if you let them send your domain to auction. Remember to cancel all your domain listing from all platform the instance it is sold or got a solid deal that the buyer can't back away without losing their deposit.
Here are some recommendation: Namejet, Undeveloped, Afternic, Godaddy domain market(remember this one only let you sell up to 1k and not over).