1) it depends on your clubs reputation: players will reach a maximum amount depending on your clubs reputation and value! if you had a player like say Javi Martinez at Athletic and he was only valued at £5m and you sold him to Barcelona for say £10m (after the time it takes for that initial price tag of the transfer fee to dissapear, usually around 6 months) you will see he will probably be valued at £12.5m
2) it depends if these players are actually getting better and playing games