World of Tanks and World of Warplanes both have 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 Xs).
A tanker can sell up to 5 (five) tanks in one day on World of Tanks.
A pilot can sell up to 5 (five) planes in one day in World of Warplanes.
This limitation resets each day with the server reset (when your x2 exp bonus comes back). At this point you can sell 5 more planes or tanks.
This limitation is true for all planes and tanks, include premium vehicles.
Keep in mind if you sell 4 planes or 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 planes or tanks.