If you're in Australia and you want to sell and buy something back, you don't get paid back on the tax you put in, so for us we have to pay an extra 20% if we want to buy back something we melted.
Bit painful, but something to keep in mind if you're in Australia, I imagine something similar might be happening in other countries too.
Not really - You pay 10% when you buy it initially, but when you melt it, this "returns" to your balance (but is not visible) and is applied against whatever you use your "credit" on to cover the tax that you would have been charged had you not used the money for the melted item, and instead bought the new item with the original cash.
Instead of thinking you are buying an item, look at it as buying $x of "pledge store credits" for which you pay a certain amount of tax. In some cases, the credits may come in the form of a "ship" (or other item), in others a "gift card"\balance increase. When melting\buying things with credit, you are simply re-allocating this "credit". A "buyback", or "warbond" purchase is simply the act of adding additional credit to your account, in a particular form.
For example (GST is 10%):
Person buys ship A at $100 + $10 GST
Person melts ship A, then buys 2 ships (Ship B at $60, ship C at $40 using credit, so no GST shown)
Person then "buys back" ship A paying $100 + $10 GST
Total paid - $200 + $20 GST
Ships owned - A, B, and C
Another person buys Ship B at $60 + $6 GST
They then buy Ship C at $40 + $4 GST
Then they buy ship A at $100 + $10 GST
Total paid - $200 + $20 GST
Ships owned - A, B, and C
Both players paid the same amount, the same GST, and have the same ships. Neither player paid "more" for a ship because of GST, or got one cheaper because they didn't have to pay GST on it. The first person didn't spend $100 +$20 GST for ship A, $60, no GST on ship B, and $40, no GST on ship C - They spent $200 on ships A, B, and C, and had $20 GST applied to the purchases.
The fact that you don't see taxes such as GST shown when using credit is because that tax was paid when the initial purchase was made - re-arranginging where the credit was applied simply re-allocates the initial tax\GST paid to the items the credit is applied to. It has no connection to the original item.