As mentioned in the rules' §4.1 you're not allowed to trade players at prices far higher than the market value. In order to be sure you are not violating this rule you would principally require a clear definition of the following.
- Exactly how much more than the market value is the price allowed to be?
- What exactly is the market value?
Unfortunately, it is very difficult to answer either of these questions precisely. Of course, we could allow up to say 25% above the market value. But that would mean cheaters could systematically perform transfers of Credits by bidding just less than 25%.
And the market value is not easy to define either. The entire economy of Virtual Manager is fluctuating all the time, and the same is the market price for each player. To get an idea about what the market price for a certain player, you should look for the player's and similar players' latest transfers.
If you are in doubt whether a certain price for a certain player is more than the what is allowed, you should ask yourself: Have you looked a similar players' transfer prices and do you still think the player is that many Credits worth? If yes, it is very unlikely you will be penalized for the transfer.