This issue happens when your account has charged back purchases.
Since such chargebacks entail financial losses for our company, we have to block such accounts. To avoid this the devteam decided to meet users halfway, and take away the amount of gold equivalent to the refunded purchase. Since some of the in-game content was already spent, the balance became negative.
In order to be able to make purchases on the market you would need to cover the negative balance.
Please note that in case of repeated chargebacks the account can be permanently banned.