I would say Sedo, seems like Sedo has a better platform. Flippa is great if you’re looking for a quick auction, however you’re up against a ton of other domains up for auction as well. It can be a hard sell at times.
Have you considered other platforms? You can actually sell your domain at the same time on multiple platforms as long as you read their term and it doesn’t mention anything regarding the exclusive right to sell your domain. The exclusive right occurs mainly if you allow them to submit your domain for auction. Remember to cancel your entire domain listing from all platforms in the instance that it’s sold or you get a solid deal that the buyer can’t withdraw without losing their deposit.
Here are some recommendations: Namejet, Unveloped, Afternic, Godaddy domain marketplace (remember this one only allows you to sell up to 1k and no more).