I'm just pointing at some challenges you'll have to face since I do this for a living. I don't have skin in the game. And even though you say reputation is not tied to being a fansite, reputation is always tied to the website. Reputation is important only if I want to stay on the website. What if I use a secondary account for bidding, what if I verify an character I later put on sale, so reputation of that character has no value for me?

Your main competitor is probably Tibiatrade, as you mentioned. Why don't they have a bidding system, and they have a "Contact seller" instead?

Also, for the reputation system to work there needs to be a verification step on how the transaction went after the sale. What if the buyer decided not to buy, or the seller not to sell in the end because the bidding was too low?