from the FAQ

Transfer Market - Why is there often only 3*/4* players on the market and no 5* players?

You are the highest level of your server (mainly a 3* market) OR the second highest level of your server (mainly a 4* market), the reason why there not many 5* players that appear on the transfer market is because there are no higher level managers above you that will want to sell of their rubbish players to you and even if they do, they are older players that are not needed by the token buyers and often result in bidding wars.