I really think the fees are high.
The 5% is fine but the base cost scaling up for longer listings should be reduced.
If the auction is to replace chest shops, the base cost is restrictive for less expensive items (build mats and food, etc.) I think 24 hours is a reasonable time frame to post goods but base fees cause a mark up that hurts the buyer and/or the profit of the seller. Again, talking about what should be lesser cost stacks.
(... truth be told, I never really understood the need for fees on a server/game auction. Who is getting that money? Noobcraft Stock Brokers LLC? The Noobstown Town Board? Are the fees needed to keep the lights in the town fountain or pay for paving the streets?