This would be cool if you could sell cars and actually flip it for a profit. That would be a pretty interesting mechanic.
If they have not implemented the ability to sell cars at launch and even for these past 6 months, then there's only 2 real reasons for that:
1. They are actually trying to develop a system way different than the past GTs, where instead of just having a button to sell the cars for 1/4th or 1/3rd of the price, you will actually get a whole menu (like the LCD or UCD) where you can sell them online to other players. And with a method like this, it does require a lot of testing and theory to make it work, so that people don't abuse it. It still shouldn't take the whole of 8 years of development of the game itself plus 6 more months after its launch to do something like this, but hey, at least here I can just blame it all on PD's incompetence, and have at least a little bit of an understanding of why they are taking so long with this.
2. They are intentionally holding back the the "feature" (I call this a feature but this should really be a default of the game really) to sell cars to make earning credits harder, and thus make the most rich or "game addict" (common terms for these is "whales") people consider MTXs to get the credits at just a click away instead of the tedius grinding.
This also helps prolong the game's longevity, as most people are forced to grind for any car they want (if they don't opt with their wallets).
It can't be anything else. If after all this time, when they finally implement this ability, it ends up being just a single button that says "Sell Car" and nothing else, then its pretty much confirmation that reason number 2 above is why they didn't implement it at launch or for the past months. Because literally all this takes to do is a slight change in the interface and programming that for any professional, is childs play to do. It would probably take 1-man hour or less to do something like this.
And for those that act (basically the GT7 apologists or the minority that plays the game in a very strict and specific way) like selling cars isn't an issue... It is.
The majority of people won't care about having their garages full of all the cars in the game. So for those people, all the gift cars they have received, some or even most of them they could've just traded them for some credits to help aliviate the grind.
Another reason is the scenario where you did grind to buy an expensive car, and ended up not liking it or finding any use for it in the game... In this case, you could just sell it back and... at least get some credits back. As it is, you are forced to be stuck with it and basically wasted 100% of those credits on a brick that will stay unused on your garage.
You want more? How about the ways you can win credits by selling an engine swapped car? For example, an RX-7 with a 1 million credit 787B engine in it? Imagine you are ridiculously lucky enough to get a few of those engines, and you can basically buy an RX-7 from Brand Central and sell it with the 787B engine for a massive profit?
And this doesn't apply to just cars... All those parts for cars you don't care about that end up becoming nothing but clutter in your parts list, when, again, you could just sell/trade those for credits that you will actually put to use on something
you want.
... Of course, let's also not disregard the obvious fact that this game has an economy, so if you can buy things, it's quite evident that you should be able to sell them as well.
Isn't PD's whole point of Gran Turismo to strieve for authenticity? Or only on the things that are just convenient for them to make a profit on? ... figures...
Anyway, it's quite obvious that there is a big reason for this to not be in the game, and it can only be one of the two I stated above. If any of you believe this is just an after thought from PD, that they just "forgot", you are being incredibly naive.
I am almost certain its all about reason number 2, but I want PD to prove me wrong on this one. And that selling cars can be an actual intuitive and engaging feature of the game. After all this damn time has passed, it better ****ing be.