Recovering Tanks with Credits



When I play wold of tanks I often need to sell lower tier tanks often so I can afford to buy tier Xs that I unlock. Some of these tanks I sold where Prem tanks and most of them just stay in the Depot forever. So I don't worry about getting them back, My T55A, KV5 and other tanks just chill there seemingly forever. However I sold my IS6 and it's no where to be found. Are some tanks not able to be bought back once sold?