World of Tanks has a built in limit on sales to protect players from losing progress due to security breaches (or the occasional little brother/sister/cat which decides to sell all your Tier X's).
A tanker can sell up to 5 (five) tanks in one day on World of Tanks.
This limitation resets each day with the server reset (when your x2 exp bonus comes back). At this point you can sell 5 more tanks.
This limitation is true for all tanks, including premium vehicles.
Keep in mind if you sell 4 tanks late at night or early in the morning, the server may have reset before your sales, so you may need to wait until the next reset to sell the full 5 more tanks.