I’ll try once more to explain what’s going on and hopefully why, though recent experience indicates that a lot of newer players to the franchise are going to happily tell you how wrong are my explanations. Ludicrous I believe is the term used.
In a real-world economy, one that functions well at lest, the total money supply is controlled. New fiat currency is printed/minted on a regular basis and old currency is removed. In the case of the US, currency is actually physically destroyed via shredding. Creation of new and destruction/removal of old generally equal each other out so that the overall money supply remains somewhat stable and fixed. This is a necessary condition as it leads to the overall stability of the economy, allowing for measured growth and increased investor confidence, There’s a reason some two thirds of US currency resides in deposit outside the US, people trust in its stability.
In very general principle, for every new unit of currency added, there needs to be an offsetting removal from the total money supply to help control the rate of inflation. Inflation being where price level of goods and services rises over time, reducing the buying power of currency. Unchecked, inflation can turn into hyperinflation where the price of goods and services skyrockets as the value of currency divebombs.
How does this pertain to Forza? The Forza Central Bank (the game itself) prints/creates new currency at a breakneck pace each day. Every race that awards credits, every wheelspin that awards credits, every daily payout that awards credits, all of these are credits printed/created by the Central Bank and dumped into the economy. Hence the need for gold/money sinks. Currency needs to be removed from the total money supply via regular means to prevent the inevitable hyperinflation.
That’s why there are fixed gold/credit sinks such as the fee skimmed off each auction, the cost of upgrade parts, the introduction of expensive houses. Then we have partial sinks in something like fast travel that comes with a fee that can reduce to zero through progression. And finally, a sort of inverse sink in the reward cars that cannot be sold back to the game itself. These cars have no direct credit value because they cannot be sold/exchanged back to game for full or partial value of buying one new. You can, theoretically, earn something from them by auctioning them off, or as suggested by someone, taking advantage of their associated perks (this is a very good idea!), but otherwise they have no value and you cannot get credits for them this time around.
All of these are design choices by the developers, some of them have been in the games before and proven to work, some are new and we’ll have to see how they pan out. Either way, they no doubt consulted professional economists when creating this economy and will continue to do so and make adjustments in future titles.